If you make a pair using the middle ranked card on the flop and one of your hole cards, then you have a middle pair. Similarly, if you make a pair using the bottom or top ranked card on the flop and one of your hole cards, then you have a bottom or top pair, respectively.
Hole cards are cards dealt to a player face down, meaning that only the player dealt those cards can see what they are. Hole cards do not need to be shown to other players until the showdown stage.
The board consists of cards dealt face up, meaning that all the players can see those cards. The flop refers to the first three cards dealt on the board.
Let's look at a few examples below to see exactly what it means to make a pair on the flop.
Example 1:
Board: 2♠ 4♣ 10♦ Your Hole Cards: 4♥ J♥
The middle ranked card on the flop is four, and you have a four in your hole cards. Therefore, you have a middle pair.
Example 2:
Board: 7♥ 9♦ A♦ Your Hole Cards: 7♦ 2♠
The bottom ranked card on the flop is seven, and you have a seven in your hole cards. Therefore, you have a bottom pair.
Example 3:
Board: 3♣ K♦ 8♠ Your Hole Cards: 5♦ K♠
The top ranked card on the flop is king, and you have a king in your hole cards. Therefore, you have a top pair.
Example 4:
Board: 10♦ J♣ A♠ Your Hole Cards: 8♣ 2♠
None of your hole cards match with any of the cards on the board. Therefore, you do not have a pair.
Example 5:
Board: 7♦ 10♠ 5♣ Your Hole Cards: 5♥ 7♠
In this case, both your hole cards paired with the board. Therefore, you have two pairs.