A full backup is a backup that can be restored independently of any backup other backup. An incremental backup links with the last backup before it and only includes changes made since.
Backup chains are links of backup sessions which start with a full backup, and then a series of incremental backups each recording only the changes made since the backup before it. Incremental backups are useful because they are fast and efficient.
Restoring an incremental backup requires retrieving the volumes of all backup sessions made before it, up to and including the full backup that started the chain. The longer the backup chain, the more time it will take to restore.