Final Fantasy XV is set in an open world, where you are free to explore wherever you can reach. You have access to a few means of transportation to help you get around, with the main one being the Regalia, your car. Although you can't drive it yourself at the beginning of the game (except for one instance), you will gain the option to drive it starting in Chapter 3. You also gain the ability to rent Chocobos as you progress in the game, up to a total of seven days.
As you explore, you will undoubtedly notice two major things, that the map is covered with a fog of war that becomes clearer whenever you pass through it, and that there is a day/night cycle. The timer displaying the time of day is in the upper right corner of the screen, to the left of the minimap. Normally, daytime lasts from around 6:00 to 20:00, which is when it will transition into night. What makes this cycle so special is that it is used for a few things, like Hunts, Sidequests, Fishing and one more thing.
At night, there are very strong enemies that can spawn by coming out of the ground. These are called Daemons and they are quite difficult to kill, as some of them are as high as level 42. It's not only the fact that you can encounter these from the beginning of the game, but when daemons inflict damage on you, they do damage to both your current HP and your max HP. This means you'll have to use items that heal max HP, such as Elixirs and Hi-Elixirs, as normal Potions won't completely heal you to full HP.
Speaking of enemies that only spawn during certain times of day, there are also enemies that only spawn during certain weather cycles. Although weather is completely random during the game, you might find that certain enemies, with one example being the Gigantoad, only appear when it's raining outside. The only two weather cycles in the game seem to be rain and sandstorms, the latter of which only occurs in the Leide area of the map, since it's the only desert and it doesn't seem like any enemies are exclusive to them.
Outposts and Camps
Scattered throughout the world of Eos, the world of Final Fantasy XV, are outposts that are safe places for Noctis and crew. You won't find any enemies here and they will usually contain a shop of some kind, as well as other amenities. A lot of the bigger outposts might have a person who will sell you food, represented as a fork and spoon icon on your map. These NPCs, commonly referred to as tipsters, offer three different services. The first service, a speech bubble, will pinpoint the locations of points of interests around the local area, including parking spots, other outposts, dungeons, camps and procurement spots (item pickups).
The second service offers you some food that you can eat, similar to whenever you rest at a camping spot, except you don't use items in your inventory, but rather pay for them with your gil. Some of these meals offered for sale can give inspiration to Ignis to create new meals at camps. The last service that is available from these NPCs are Hunts, where you will be ordered to go out and kill a certain enemy or group of enemies. Once you do this, you will return to the tipster to receive your reward, usually consisting of gil and some items.
Modes of Transportation
As already mentioned, there are two main modes to get around in the open world, the Regalia and Chocobos. The Regalia is very useful in travelling large distances, with the option to either let Ignis drive and once you've gotten to Chapter 3, manually drive the car. You will unlock parking spots in the game's world as you explore and as long as you choose to park at them, you can use those same parking spots as fast travel points. Going around and getting as many parking spots as possible will make it much easier to get around later in the game.
The Regalia can be customized by heading on over to Hammerhead and speaking with Cindy. You can change the body color, but this will require you to find and loot ores and other such things in the world. Other things that can be customized include adding decals to the front, back, sides and top of the vehicle, as well as changing the color of the interior and choosing different sets of wheels. Note that the Regalia upgrade you can get in the postgame is separate for customization than the normal Regalia, so you actually get to customize two different vehicles.
The other mode of transportation is the trusty Chocobo, which can be gotten as early as the beginning of Chapter 3. The only problem is that you can only rent Chocobos, up to a maximum of seven days. If you're riding a Chocobo whenever your time expires, then the Chocobo is returned as soon as you get off of it. As you're riding the Chocobo, it will level up, to a maximum of ten, which grant the Chocobo various bonuses as well as some abilities that can be used in battle. Similar to the Regalia, you can customize the color of your Chocobo by doing the sidequests given out by Wiz, as well as finding specific items during the Chocobo races.
Throughout your adventure, you will sometimes come across people who need your help with something or stumble across a quest that isn't part of the main story. Usually, these are marked with an orange "?" on your map, although not all of the sidequests are marked with this and will only appear as you get close enough (ex. the ones associated with optional dungeons only go into your quest log whenever you first enter them). Completing these will reward you with a lot of experience, gil and even some nice items, so it's in your best interests to do them.