They play whenever they break a tie.
For example, let's say your hand is a pair of kings. That's just two cards... and a poker hand is five cards. So, if this is the board:
And you have:
What is your hand?
It's not really just "kings." It's
- it's kings plus the three highest cards besides the kings.
Now, if someone else has
, then their hand is two pair... so you high cards don't matter. Two pair beats a pair.
But if their hand was
, then they have a pair of kings, and "nothing else,' just like you. They have
. And here, you queen "plays," breaking the tie, giving you the win. The queen is always there, but she only matters if there's a tie.
Of course, if they had
, their hand would be
- the exact same hand as you. In fact, the queen and the jack and the seven are all "playing," but it's still a tie, all the way.
It's sometimes called a "kicker" when the card is in your hand, with the pair card, but whether or not it's a kicker, it's a "high card."
Now, imagine the board was:
You have:
. Your full hand is a pair of kings,
And they have:
. Their full hand is a pair of kings,
Again, you both have kings, but it comes down to the very last card... your six beats their five. Your six plays as a "high card," because it's higher than their five.
Does that help?
Is there a particular kind of hand where the rule confuses you?