Speculative hands are hands that do not figure to be winners at the moment but that can improve to be very powerful later in the deal. Examples are suited aces or small to medium pocket pairs. These hands, in hold 'em, aren't very good preflop, but if the flop comes down right they can produce powerful hands or draws such as sets for the pairs and flushes for the suited aces.
The key to speculative hands in hold'em is usually to see the flop cheaply. Usually when the flop does not hit you hard, you're going to fold to any resistance.