Scott has been using Dropbox as a backup, but he's realized that Dropbox doesn't remove the files from his computer. Leo says that's because Dropbox syncs to the computer. He could delete the files, but sync will then delete them from Dropbox as well. Scott would have to backup to Dropbox and then remember to turn off the Sync feature. Then before he turns sync back on, he'd have to copy those files to another backup source, otherwise it'll sync deletions.
Sync means making two computers the same. Having a solid backup like Carbonite is a better option. Even better, and cheaper — he could buy an external hard drive and drag the files onto it. That way they are backed up.
(Disclaimer: Carbonite is a sponsor)