Uuid's in plain English:
uuids as a term: Universally unique identifier

UUIDs in minecraft
UUID's is a new method minecraft introduced, it will come By the 1.8 update of Mc, but lets see how they are used..
on 1.8 minecraft you will be able to change your name! But you may say "hey! What will happend to baned players! If they change their name, they will no longer be baned." And exactly because of that minecraft gave to each player a UNIQUE id that will be used by plugins instead of players name. If a player changes his name the I'd won't change and he will be still baned if his I'd is baned.
Soo let's recap, a UUID is a Unique I'd for each player.

What you should be CAREFULL of if you own a server.
You Have to update all ban plugins to a plugin that supports uuid's
And generaly each plugin that stores information about players should be updated to a version that supports uuid's, if you have a custom made plugin and you can't have it's developer to update it for you, I can help, just post a comment Or pm me :D