Think of bitcoin as a material which is sent as powder collected in sachets. Each sachet is locked by a key-protected mechanism in a way that, when it is unlocked, the whole sachet gets destroyed and can not be recovered.

At that time you need to collect (almost *) all the powder in one ore more different sachets and lock each of them with a similar mechanism to the one you just opened (of course, the key for each of them may be different).

UTXO = the intact sachet, still locked

Powder (or better, indivisible atoms) = satoshis

Transaction = the process of destroying UTXOs and create new UTXOs with satoshis

* = some satoshis need to be left on the table for the miners' vacuum cleaners, in order to tempt them and confirm the transaction for you.