You're on a car trip with your son. He says that he has to go to the bathroom, but you just stopped at a gas station 10 minutes ago. You don't want to stop again so soon. You ask him:
Can you hold it in for a few more minutes?
When you have to use the bathroom, but you need to wait, you "hold it in".
Something else that people "hold in" is a secret:
She told me not to say anything about it, but I just couldn't hold it in!
So you might say that people "hold in" something that's inside of them but pushing to get out.
This is an easy expression to understand, but it might be helpful to remember it all together. Here are some other examples:
Stir it for a few more minutes.
Let's wait for just a few more minutes.
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