First, let's define a top pair. You have a top pair when your pair contains the highest card on the board. By example: you have in your hand, a 9-10, and the flop is 4-6-10. You have a top pair because of your pair of 10's.
If instead you had 6-K in your hand, with the same flop of 4-6-10 you would have a pair but it would not be the top pair since someone could have a 10 in their hands giving them the top pair.
I advise to fold if after the flop, you do not have a top pair (or better) and there is a bet in front of you. 'Or better' above means having three of kind, a straight or flush.
