If you have ever played Persona 3 or Persona 4, then you will recognize Confidants, which is just another name for the Social Links in those other games. They are essentially important NPCs that you can hang out with during your time in the game, building relationships with them. Doing this has two major advantages for you, the first is that it provide your Personas with a boost in experience whenever you fuse them (the experience earned is based on the rank of the Confidant). The other benefit is that as you level up a Confidant, you will gain abilities that will help you out on your journey, such as party members learning useful abilities for battles.
Things are exactly easy, though, as you will need to plan things out a little bit. Confidants aren't available every single day, as some might not like the rain, but you won't get full access to them right away. As you play through the calendar year, Confidants will regularly become available, although some might have some requirements, such as requiring a Social Stat to be at a certain rank (ex. Charm at rank 3). This requirement sometimes extends to ranks while leveling the Confidant themselves. You will also find that some Confidants will give you a Mementos request and won't be available until you complete it.
Leveling up a Confidant is as simple as hanging out with them, but it goes a little deeper than that. Usually, you will hang out with a Confidant and they will have you respond to them, with your character given a few different responses. Picking the right responses will elicit some yellow notes to pop out of the Confidant's head, which is your cue that you've picked the right one (up to three notes can appear). These will help towards your next meeting, but sometimes, you don't get enough points for the next rank event to occur.
This will make it so the next time you hang out with that Confidant will not result in a rank up, but rather, you will hang out with the person with no real dialog. Once you progress in the game and open up some other Confidants and locations, you will be able to spend money to advance past these non-ranking sessions, with one method not even taking up time.