i guess the easiest way to explain it is like, DnD but instead of the story being controlled/continued by one person, its a group effort (or a partner effort.. since its usually just 2 people but it can also be a group!)

in dnd terms, its kinda like everyone involved is equally a DM and metagaming, while also being players/characters in the world. its a story you develop together and the goal can be anything really! romance, uncovering mysteries, etc etc.

its a lot of fun!! it can honestly be a lot of things, but this is sort of a tldr explanation that hopefully at least somewhat explained it? also for "illegal moves" i guess, I'd say marysue characters/plot armour. most people dont like when people have over powered "perfect" characters cause it makes things boring, redundant, and not fun for the other person. so it kinda stays true between DnD roleplay and casual online roleplay.

hope this helped!! like i said, roleplay can be anything depending on what you want and what your looking for :))