In no-limit hold'em, the term "slow playing" doesn't typically refer to players thinking about their decisions for a long time and playing their hands too slowly — although that is a commonly discussed topic these days, especially in tournaments. Rather, slow playing usually refers to a player playing a strong hand weakly in order to encourage opponents to stay in the hand and hopefully contribute more chips to a pot the player expects to win.
If you've heard the term "sandbagging," the idea is similar. A player who flops the nuts watches his opponent lead with a bet, and instead of raising he just calls or "sandbags" in order to disguise the strength of his hand.