what is griefing and why am i banned from the server?

hello can somone explain to me what griefing is i was told i was banned from the server because of griefing im new to minecraft etc. and when i came home from weekation it told me i was banned for griefing and plz i havent done anything and want a unban this was my favorite server so far :S

Griefing is destroying a user-made structure. It's against the rules. To prevent being banned by this, don't destroy or build on what isn't yours.

okay but i havent done anything i havent even ben onlinge for 4 days or something so when i logged on yesyerday/in the night i was banned and i dont knwo ehat to do :S