You and 4 friends each put a 10 dollar bill in a pile. You mix up the pile and you each take one 10 dollar bill out of it. It's now not certain who has your initial bill. (Forget the unique IDs on dollar bills for a second). Not a perfect explanation but it's easy to understand.

In Bitcoin, each UTXO has a tx history. When you join a CoinJoin pool, you mix up your UTXO with other people's UTXO's in a similar way. No one knows who ended up with your initial UTXO, and no one knows which UTXO now belongs to whom.

CoinJoins are however very visible on the blockchain. If you're interested, you should look up CoinSwap. Chris Belcher is working on this feature, which would allow people to swap coins on the base layer without any trace (it looks like a normal tx).