The UTXO set is the comprehensive set of all UTXOs existing at a given point in time. The sum of the amounts of each UTXO in this set is the total supply of existing bitcoin at that point of time.

Bitcoin is special as a money in that anyone can verify the total supply at any time in a trustless manner. All nodes maintain identical copies of the UTXO set. When a new block is created, the UTXO set is updated as some UTXOs were spent and new ones were created.