When you send a bitcoin transaction it goes to mempool first (if it is valid). It's then unconfirmed.

When a miner chooses to include it in a block it becomes confirmed (once).

Each time a new block is mined after that and build on top of the first conformation it gets confirmed again.

Sometimes a Blockchain gets reorganised and a confirmed transaction can become unconfirmed.

This can especially happen within first 2-3 confirmations.

It's highly highly unlikely to reorganise after 6 confirmations.

That's why large amounts should have 6 confirmations before being treated as settled.