A UTXO is a spendable piece of Bitcoin. You could have a piece for 50BTC for example. But if you spend it for 10 BTC now you have two pieces, one for 10 and one for 40. Now you could make millions of pieces with 1BTC in theory.

Check this out: https://bitcoin-bumblebee.com/2023/04/13/bitcoins-utxo-model-and-implications-for-user-privacy/