A "sleeper" is someone whose perceived value is lower than what it should be, meaning he could become a steal in the NFL draft. These guys may slide to Day 3, and some even go undrafted.
Ultimately, they can sometimes be as effective as "premium" players.
Be it poor athletic testing, lackluster production, unrefined skills or a difficult scheme projection, prospects fall for a number of reasons. What sets them apart from the rest of their Day 3 counterparts is they possess a trait or profile that suggests they can overcome their low draft slot.