In fantasy football, many people want to protect their top investment in a running back by handcuffing them. This means taking their direct backup in case the starter gets hurt and has to miss time. This way they have the replacement on their roster without spending a lot of FAAB (Free Agent Acquisition Budget) money on the waiver wire or using a high priority pick in waivers.
There are different viewpoints on handcuffs. Some think it's a waste of a roster spot. If the starter never misses a time, then the handcuff will never contribute for your team.