When you post a record to a database (with the function rec_post() for example), you place it in a queue that holds records that are to be written to the database until the function db_update() is called.
A set of records posted to the queue is called a transaction. When the database is updated (with db_update), DP4 either writes the complete transaction to the database or rejects the whole of it. The next transaction starts the next time a record is successfully fetched from the database or when another update function is called.