could for possibilty

[akdokme]

What does could for possibilty mean

[Casiopea]

could (used to express possibility)
EX: I wonder who that could be at the door.
EX: That couldn't be true.

could (used to express conditional possibility or ability)
EX: You could do it if you tried.

Source: http://dictionary.reference.com/search?q=could