(something) is 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 this example:
Hey, um, you've got something stuck in your teeth.
...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.