RPG stands for Role Playing Game.
The quickest way to describe it is “Make believe with rules and dice.” There are some RPGs that don’t use dice, so they can be described as “Make believe with rules.”
If you ever played Cops & Robbers as children, you know how to play make believe.
RPGs take our ability to pretend to be someone else and give it a context, i.e. fantasy, science fiction, steampunk, etc., and rules.
The rules describe what your character can do and different games have different ways of doing that.
D&D is the best known example. You are a player and you generate a character. You roll dice to determine the character’s strength, intelligence, wisdom, dexterity, constitution, and charisma. Based on the scores for each ability, you can choose different races, human, elf, half-elf, dwarf, gnome, halfling, and half-orc; and different classes: cleric, druid, fighter, paladin, ranger, magic-user, illusionist, thief, assassin, or monk.
You then determine a name and depending on the style of play, you develop a back story, or just start playing.
The level of role playing is up to you. Some people are more comfortable saying what their character does in the third person and do not have a voice to go with it. Others will talk differently and say things in the first person, as if, they were that character, much like in cops and robbers or an actor on stage or screen.
The key to RPGs is having fun. There may be several rule books over hundreds of pages, but if the mechanics of following the rules gets in the way, toss our what isn’t fun. If you are not having fun, then you aren’t doing it right.