Change is basically the amount of XMR that gets sent back to your wallet. Let's say you received an output of 10 XMR from an exchange, a service, or someone who you traded with. Subsequently, if you make a transaction, of 5 XMR, the Monero GUI software will use that 10 XMR output as input. Thus, you have a transaction with 1 input (10 XMR) and 2 outputs, i.e., a 5 XMR output (minus the fee) that goes back as change to you (the sender) and a 5 XMR output that goes to the recipient.
If you have trouble grasping the concept of inputs and outputs, think of them like bills. Let's say you received $20 from your grandma for your birthday. This $20 is the output you initially receive. Subsequently, you use that $20 to buy some groceries worth $10. You hand the cashier your $20 bill and will get $10 back from him/her. In this "transaction" the $20 bill is used as input, the $10 you have to pay for groceries is the output that goes to the recipient, and the $10 change you get back is the output that is sent back as change to the sender.