You might have multiple inputs in your account, in many different denominations (For example: you deposited 0.5 XMR on Friday, and 0.75 XMR on Saturday). So, when have a transaction with an input of 0.5 XMR, but you only want to send 0.1 XMR, your transaction will include a fee to pay the @miner, an output for 0.1 XMR to send to the recipient, and the rest that you want to send back to yourself will be an output back to you (this is called "change"). Once the transaction is completed, the change becomes available to you as an input that you can again split and send with a new transaction.