The wallet that you are spending from has more amounts of bitcoin than what you are sending and the difference is returned to your other address as change minus the miner fees.
A 'wallet' is a piece of software which manages your private-/public- keys.
Each time you receive BTC, you get one 'bill' cointaining the amount you have received.
When you try to send some BTC your wallet takes as much 'bills' as necessary and 'destroys' them in order to create 1 'bill' for the actual transaction (the amount you want to send) and 1 'bill' for your change.
Also explained
here.