While the execution of the transaction, such checkpoints are marked, and the transaction is executed then using the steps of the transaction, the log files will be created.
When it reaches to the checkpoint, then the transaction will be updated into the database, and till that point, the entire log file will be removed from the file.
Then the log file is updated with the new step of transaction till next checkpoint and so on.
The checkpoint is used to declare a point before which the DBMS was in the consistent state, and all transactions were committed.