What's "heat up" mean?

I saw this expression "sleep in" in the movie Home Alone. What's it mean?

To "sleep in" means to sleep or stay in bed longer than usual! Ex: I slept in today. It was so nice getting up later than seven am. That's when I usually get up. Ex: On the weekends, all my friends like to sleep in. So we prefer to go out at night.

What does "benign" mean?

"Benign" means pleasant or kind; harmless. In medical terminology, a benign tumor means it's not cancerous. Ex: The cat was very benign. Ex: Her doctors told her that the tumor was benign.

What does "yourself" refer to here? and shouldn't it be "themselves" not "yourself"?

You are correct, the speaker misspoke here. She should have said "themselves" as she is referring to the action made by the pizza makers themselves. Using "yourself" here is wrong. Or, she could have said "you can come and see them make the dough yourself," or "you can come and see for yourselves them make the dough" to refer to the action of personally seeing the dough being made.

What is the difference of "out in the real world" and "in the real world"?

"Out in the real world" refers to experiencing life without a certain boundary, like school, work, etc. Ex: Out in the real world, nobody is going to help you with this. "In the real world" could mean the same thing, but it is also used to suggest that someone is living in a fantasy and trying to break them out of it. Ex: In the real world, you will never be able to afford that.

Does "running down" mean "flowing down"? And please explain the other usage of "running down".

Good question! Although in this context, 'flowing' and 'running' are both verbs, there's a slight difference between the two. 'Running', in this case, is used to describe rapid water or liquid movements. While 'flowing' means more of a steady and continuous movement. Ex: The flood water was running down the street. I love the feeling of Cola running down my throat. The water was running down him.