As you go along your journey on Pokémon Sun and Moon, you will bump into new combat moves and you get the chance to learn them and eventually use them. However, you have to remember that the moves you will learn at higher levels are necessarily better than the others because in this game, what you need to consider is play style and the foundation of your team.
Technical Machines, or shortly called TMs, is one best way to organize the moves on any Pokémon. There are 100 TMs scattered throughout the game and each of them has different skills for your Pokémon to learn. You have to remember that there are some moves which are not available through TMs. Fortunately though, Pokémon Sun and Pokémon Moon both have two NPCs to help you-the Move Reminder and the Move Deleter.
Pokémon Sun and Moon - Where To Find Move Reminder NPC
Like on any other Pokémon games, Move Reminder teaches your Pokémon a move that it either had as an Egg Move and has forgotten, or as an innate skill it gets after leveling up. It is helpful to guide you in using the correct moves or if you removed a move by accident. In Pokémon Sun and Moon, the new feature that comes along with it can teach you a move normally learned at a higher level.
You can find the Move Reminder NPC later on in the game at the Pokémon Center in Mount Lanakila, Ula'ula Island.
Pokémon Sun and Moon - Where To Find Move Deleter NPC
On the other hand, Move Deleter is used to allow players to remove HM moves. With HM no longer on Pokémon Sun and Moon, this NPC is less useful. Even so, if you want to remove or delete Pokémon's moves in whatever reason it may be, you can locate him at the Pokémon Center in Hau'oli City.