CoinJoin is a layer built on Bitcoin’s blockchain with the aim of introducing more privacy to transactions.
The process requires finding different Bitcoin users who want to mix their sats and create a joint transaction. With this transaction, the users send their respective Bitcoin to an address in identical amounts and then receive the same amount of Bitcoin they sent back (minus mining fees).
The transaction then sends the Bitcoin back to these users in identical amounts, which would make it impossible to differentiate each user based on the CoinJoin transactions.
For a more detailed explanation of how CoinJoins work, you can read the article by Che here.