Unspent Transaction (TX) Output

A claim on some amount of bitcoin. All are viewable on https://mempool.space or by using your own node. 

It's a timechain entry that has yet to be used as an input for a transaction. Once used, they are consumed and new ones are created. If you're not careful, you can create a permanent UTXO that cannot be spent. This was easier to do early on but modern wallet software avoids this now.

UTXOs have sizes, value size and byte size, and are part of how you determine the fee paid in a transaction. The more UTXOs as input, the more you'll pay, since you pay for every byte of data. 

https://mempool.space/tx/8cee4fb204c0aef97b59ad5e3827bd1caa5780c2671f36ebe82fcf3dd746e9cb?mode=details

This one is a good example of a relatively simple transaction that took a ~3.9BTC UTXO and created three new ones. ~8k sats to the miners as a fee (part of something called the "coinbase transaction" aka how new Bitcoin are issued to miners). And ~2mil to some address, presumably the person being paid. And then a "change" output back to themselves. They used poor address security here by reusing the same address for input and for change so this might be an exchange sending out someones sats. Most quality wallet software will never reuse an address, which helps with privacy.