Can someone explain what is meant by 'Roll Forward' in the following description please ?. Does it mean that they are discarded or commited to the data file?

When SQL Server is restarted, it looks for the most recent checkpoint in the transaction log and rolls forward all transactions that have occurred from that point forward since it is not guaranteed to have been written to the data file until a checkpoint is entered in the transaction log. This prevents transactions from being lost that were in the buffer but not yet written to the data file.

Thanks.