Slow playing is a common practise in poker however new players or those who are just learning to play often attempt to slow play a hand more often than needed. Slow playing is showing your opponent that you have a weak hand when your hand is strong and either calling or checking the hand instead of betting out first or raising. The reason you would call or check is you want as many player as possible to stay in the hand maximizing the pot by allowing those player to bet their hand for you.
One of the problems with this is players will often overlook that they are giving players odds to draw out cards and play the hand to the river. This leads to hands being beaten when in other situations they would have won because you are giving away cheap otherwise known as free cards to your opponents.
In a case where you have provide a cheap hand to play out your opponent could be playing bottom pair and because you have allowed him to see the turn for next to near nothing or even worse for free, they have now won all of the best and blinds.
Another situation that a beginner finds themselves in often is slow playing someone who is actually slow playing them. This means playing second best hand into the nuts.
A problem with this scenario is you could end up with a very small pot if your opponents are not betting for you. This may mean that they could be slow playing you with second best. In a case where you check and your opponent checks it would be best to place a bet on the turn. The bet should be enough to keep them in and not to low that players with an even worse hand than the two of you to call and out draw you both.
Something else to consider is how you slow play. Far too often a novice player will slow play their strong hands and bet their weak hands which are quickly picked up by stronger players. Just like bluffing you should slow play hands sparingly and pick the right time to do it.