Literally, this translates to "you have a face". Well, doesn't everybody?
But of course, if you've studied Spanish long enough, you know that you can't translate everything literally. So with that in mind, what does this mean?
Well, the expression "Tienes una cara" is used when someone has a look on their face as if something might be wrong, maybe they have something on their mind, they just don't look like they normally do.
This brings us to a very similar expression "por qué esa cara?" - "Why the long face?" might be a good translation. You use it when you see someone is clearly bothered by something. You might also follow up this question with another one..."qué tienes?" - What's wrong?
But let's get back to our discussion of caras.
It turns out there are many different types of caras.
Tienes cara de enojo/enojada/enfadado.
You look angry.
Tienes cara de sueño.
You look sleepy.
¿Por qué tienes esa cara de niño con juegete nuevo?
Why do you look like a kid with a new toy?
¿Por qué tienes esa cara de felicidad?
Why do you look so happy?
Tienes cara de sorpresa.
You look surprised.
There are many, many more types of faces in Spanish, and this is just short list to help you get started talking about them.
¡Hasta la proxima!