Definition of Incremental Backup in the Network Encyclopedia.
What is Incremental Backup?
Incremental Backup is a backup type in which only selected files and folders that have changed since the last normal backup are backed up and the archive attribute is marked for each file and folder.

(graphic from easeus.com)
You can use incremental backups in conjunction with normal backups to simplify and speed up the backup process.
If you do a normal backup on one particular day of the week, you can perform incremental backups on the remaining days to back up only the files that have changed during each day of the backup schedule. Incremental backups are faster than normal backups and use less tape.
Incremental backups are not cumulative
Incremental backups are not cumulative, as differential backups are, so when you need to perform a restore, you need the normal backup and all incremental backups since the normal backup was done. Incremental backups are faster to perform but take longer to restore from.