An online text-based role-playing game is like playing make-believe on the computer with other people. You get to pick your character and what they look like, and then you tell a story with other people who are playing the game too.
Instead of pictures or videos, everything happens in words on the computer. It's like you're reading a book and making up the story with other people at the same time. You type out what your character is doing and saying, and the other people will give you responses as if they are also in the story with you!
It's like writing a story with your friends, only you're doing it together in real time through the computer. The fun part is that you get to create your own story, use your imagination, and have adventures with other people. In some games, you can save the story so you can keep playing the same character and story again and again.