A full backup is where you copy all of your data to a backup location such as an external disk or network drive. This backup will be an identical copy of the original data you have, and will take up the same amount of disk space as the original data, unless compression is used.
Most people will want to keep their backups up to date. If a new full backup is made each time an update is run, over time it will consume a very large amount of space, and will take as long to run each update as it took to run the first backup, since all the data will need to be copied each time.
For example, if you have 50GB of data to backup and you schedule it to run once a week, then after 6 months you would have 1.3TB (50GB x 26) of data, most of it redundant.