As you go off the beaten path or progress deeper into the game, there will be occasions where you'll encounter an obstacle than you can't pass without the aid of Pokémon. At these times, you will need to use the corresponding TM (Technical Machine) or HM (Hidden Machine) learned by your Pokémon.
Note : Although you can freely use HMs in battle, you can't use them outside of battle - to clear obstacles - until you've acquired a certain Gym badge.
Also beware that once learned, HMs cannot be forgotten (until you reach the Move Deleter later on). This can be annoying if you have a lot of great moves on your Pokémon and you're forced to delete one when learning a new one (although luckily there's also the Move Reminder who lives in the same house).
For this purpose, some players like to bring along a "HM Slave" (or two) - a Pokémon whose only purpose is to use HMs. This way, you don't have to compromise your battle Pokémon, but it means you'll be wasting one of your six party slots. Man... when will Game Freak finally remove HMs?!
Acquired from the Parfum Palace back garden, this move lets your Pokémon - with sharp claws or a scythe - cut down a thorny plant blocking your way to a secret path or goodie. In battle, its damage is pretty poor and outside of battle, there are few (if any) thorny plants in the way of progress.
TM94: Rock Smash
Obtained from Ambrette Town ; Rock Smash does... exactly what it says, breaking small, cracked rocks in the way. It's no longer a HM, so can be relearned or forgotten as often as you like. Since it's been demoted to TM status, that's a hint that it's not vital for getting around Kalos.
However, besides opening up side routes, Rock Smash occasionally lets you encounter wild Pokémon hiding inside the rocks, items or - after beating the Elite Four - even Pokémon fossils (rarely found in Glittering Cave)!
Given to you when you step foot into Route 12 and after obtaining the Mega Ring; Surf lets you traverse medium-depth water, such as rivers or ponds. While surfing, you may encounter wild, water-based Pokémon. Of all the HMs, Surf is one of the better ones for battle and ideal for your special Water attackers.
Found in Cyllage City ; Strength flexes your Pokémon's muscles, allowing you to move big cubic rocks, provided there is no other obstruction in the way (such as a wall). Cue a variety of Strength-based puzzles, where you must cleverly manoeuvre the rocks, often dropping them into pits, to get exactly where you want.
Once you've used Strength once, you will be able to continuously push rocks until you leave the area, so there's no need to keep applying Strength each time you want to nudge a rock.
The final HM is given to you near the end of Route 19 ; with this you can ascend and descend the giant waterfalls encountered while using Surf. Like Surf, Waterfall is a powerful move in its own right and a good fit for any physical Water attackers.