Abbreviation of unspent transaction output. Think of this as a piece of money.
Once you wrap your head around what an Unspent Transaction Output is, your understanding of the Bitcoin network will really take off because Unspent Transaction Outputs is to use a legacy system phrase "Money in the Bank".
The Bitcoin network is an open ledger of addresses and their balances that everyone who runs a Bitcoin node keeps a copy of.
The UTXO's that you hold the keys for are your possession of value. You are able to decide when and how much to transfer to another user if you want to, or you can keep saving them.
The same decisions that go into when and why to send money out of your bank account come into when and why to send a UTXO.
Different Bitcoin wallets will let you have more or less control over your UTXO's depending on how easy or advanced of an experience you would like.