Explain Like I'm Five

Tabletop Role playing Games are games set in a conversation.
you and your friends talk, telling a story and in doing so, observing the rules et of the chosen game.

This is different from just collaborative storytelling because, when it is doubtful wether an action might succeed, you roll dice to decide. When to roll dice will be part of your chosen set of rules.

Dnd and many other traditional TTRPGs divide roles: one person is a Narrator (dungeon master) while each other person plays one of the protagonists (player characters). There are also games that swap role of Narrator, let you play all by yourself without one or any other constellation. Dnd is a classic game with a Narrator.

Now, if you and your friends decide to play, you will need reference to the rules. With help of these, you can craft the player characters, and the Narrator can think of a compelling story to drop these characters into. You then talk about the fictional situation and then the players controlling the protagonists decide how their characters react to the situations the narrator lays out. When something is easily done, ie opening an unlocked door, you just say that you do it. If something is hard and success is dubious, like rappelling over a ravine, you might be called to roll a dice. While playing dnd, this will typically be a 20 sided dice, and you will typically add some modifiers based on your character.
your result will then tell you wether that action succeeded or failed. This changes the narrative, so you take some time to describe what happens and then move on with your conversation-story.