um

People say "um" when they don't know what to say next, or when they're a little nervous about what they're going to say. In the example above, the speaker is worried that it might be a little rude to point out the food stuck in his friend's teeth. But since he says "um" at the beginning, it's clear that he's not telling her that to make fun of her or be rude. "Um" makes the sentence softer and more polite.

you've got something (somewhere)

"You've got something ___" is a phrase that you use to tell people that there is food or another substance somewhere on their bodies that it doesn't belong. For example:

Hey, you've got something on your lip.

You've got something in your hair.

"You've got something ___" is similar to "you have something ___". Both are OK for normal conversation, but I would usually say "You've got something ___". "You have something ___" sounds just a little more formal.

stuck in (somewhere)

To be "stuck in ___" means that something is in a place and isn't coming out very easily. A person can be "stuck in" a room:

The door wouldn't open, and I was stuck in the bathroom for 20 minutes.

In the example above, the speaker says "stuck in your teeth". Obviously, the food isn't inside of the actual teeth. Instead, this sentence means that the food is in between some of her teeth and isn't coming out.

Some other common places where things get stuck include:

I was stuck in traffic for over an hour.

There's something stuck in my throat.

Our car got stuck in the mud.

I feel like I'm stuck in a rut. (This means that you keep doing the same uninteresting things every day.)

Hey

You say "Hey" at the beginning of a sentence in casual English when you want to get someone's attention. For example:

Hey, um, you've got something stuck in your teeth.

It's common to say "Hey" before you ask someone a question, or when someone has done something that's wrong or unfair:

Hey, you're taking up the whole bed!

Use "hey" with people you know or are familiar with. When you're talking to people you don't know as well, "Excuse me" is more polite.