If you have 2 inputs: 1 BTC on input A and 1 BTC on input B and you send 1.5 BTC total to a third party, the extra 0.5 BTC doesn't remain on your original inputs, which are considered spent, it becomes a new input to an address you own. This new input is known as change.

So inputs A & B are no longer yours, the blockchain considers them spent and they can never be reused. The only way for you to access the remaining balance is by creating a new transaction from the change input.

I didn't understand this until I finally read Satoshi's paper. It's pretty easy to understand the important points even if some parts are lost on you as long as you understand the basics of cryptographic signatures