Say you have 1 BTC on an address and you want to send 0.1 to someone, you still need to send all of the money. So wallets "split" the 1 BTC into 0.1 and 0.9 outputs, sending the 0.9 to yourself to another address you control. It's called a change address.
Modern wallets do this automatically, but it can be confusing to look at it on a blockchain explorer.
Say you have 1 BTC on an address and you want to send 0.1 to someone, you still need to send all of the money. So wallets "split" the 1 BTC into 0.1 and 0.9 outputs, sending the 0.9 to yourself to another address you control. It's called a change address.
Modern wallets do this automatically, but it can be confusing to look at it on a blockchain explorer.