When a player has a top pair hand combination, one of their hole cards matches up with the highest card on the board.
For example, if your starting hand is AQ and the flop comes Q 7 5, you have a pair of queens, which is a top pair in this situation. On top of that, you have the best kicker, so your hand can be characterized as top pair top kicker or TPTK for shorts. No one can have abetter top pair hand, so your holding is generally very strong.
Of course, you can still lose to other hand combinations like an over pair, two pairs, or a set, but you should be looking to build the pot when you have this reasonably strong hand.
It is worth mentioning that the kicker plays a huge role when evaluating your hand strength. If we go back to the previous example where you have an Ace as a kicker, you can see that you will win against any player that is also holding a queen but a lower card next to it. On the other hand, if you had Q8 suited in that hand, you would lose to most top pair combinations, including AQ, KQ, QJ, QT, and Q9.
While the top pair is a strong hand on dry boards and will win you plenty of pots, it becomes significantly weaker when you have connected community cards. If you are holding AsJs on Jc Tc 9c, you are already losing to all flush and straight combinations and can be easily outdrawn on turn and river even if you are ahead, so the hand works mostly as a bluff catcher.
It is essential to understand relative hand strength based on board texture, what kind of action took place before, stack size, and even the number of opponents. While it can be a fairly lucrative hand heads-up, the EV diminishes when you have multiple players.