Bitcoin transactions have inputs (btc being spent) and outputs (where the money goes).

You can think of your btc balance as a bunch of (digital) gold coins. If I send you 0.1 btc you now have a 0.1 denomination coin, and if I then send you 0.2 btc you now have two coins: 0.1 and 0.2

If you send 0.25 to someone else, its the equivalent of melting both of your coins, and minting a new 0.25 denomination coin and a 0.05 denomination coin. You send the former and keep the latter.

Coins, are UTXOs because they're the output of a previous transaction that generated them and have not been melted yet (thus, unspent).