What are Motivated seller leads?

[Anthony Johnson]

My knowledge of real estate investment is still very basic, if someone could explain "Motivated Seller Leads". My thought process of buying and selling is very linear right now. If someone is "Motivated" to sell, but are unable to, why would you by, unless it requires rehab and is well below area value... If explanation is too lenghty, referral to any books or web references would be greatly appreciated.