Each time a backup is performed a check is carried out. This check looks at whether the files in the backup selection have changed since the last time they were backed up. If the files haven't changed, then they haven't uploaded again.
This means only new and changed files are uploaded in each backup performed. This process can also be applied on a file level. When a file changes, only a fraction of the data is different. Just Cloud has the ability to operate incremental file backups, this means that only the changed parts of files are uploaded in each backup performed which makes the backup even quicker.
Enabling incremental backup is carried out in Settings > Backup Strategy