RP is the abbreviated form of Role Play which literally means to play a role/part. Playing RP, as we say, is giving your character a real life including a history, creating him a personality, a style etc. Playing RP is a bit like acting in a play with no precise scenario but with a live scenario building up as life passes by with the other present persons. To make it all clear, in a RP game you embody a character, but you are NOT him. You're just like an actor. You play a part in a fantasy world.
The character you embody won't look like you, won't have the same personality, won't have the same attitude, qualities and failings, ambitions and certainly won't have the same background. It's up to you to determine who you want your character to be and to do everything you can to make it happen and create a fantastic, unique character. While creating such a character, try not to stick to your own personality and experiences too much.
Be inventive and make the most out of the acting! This way, your character can be very different from who you are. However, it can also be a copy of your own personality if you're not too inspired but don't forget to adapt it to the game universe within which your character will evolve.