How to reset settings then restore ipad with a backup

Apple told me to backup my ipad using iTunes then reset settings (I think not erase as well). They said then use this to restore it and it will remove a glitch which has made the ipad block off 2/3 of its memory for video use, the store said it may have done this intuitively. I fear about what I can lose and when I plugged my ipad into my computer to backup it was far from clear what to do. Can explain further what to do, I got the feeling it it a common problem given i did not need an appointment to have it explained to me.