A good news is that there is great documentation that you can read on-line. Note that it's quite a lot of information and it's super-critical for your client's data, therefore you should pay a lot of attention to it. You may also use additional resources, such as video courses for DBAs.
When something fails and you need to restore the database, you'll start by restoring the last backup. But because you make a backup just once a day, it would mean losing up to one day of client's business data, which doesn't sound good. Fortunately it's not your only option.
Because the transaction log contains information about all transactions happened since the last backup, you can apply it again to get into a point just before the failure. Obviously if there was a lot of transactions since the last backup, the file will be large. It also means that the restore will take a lot of time, which isn't good either.
The way how you can reduce the amount of data since the last backup is making backups more often. As you can see in the documentation I mention above, full backups are not your only option. Typically there are several differential backups between full backups and very frequent log backups.
