It's a word I had seen many times, and comes from fijarse, but I never could quite get a handle on what it meant. That's partly due to the fact that "fijate" has many definitions.
Today we'll look at a few commonly used definitions of "fijate", the first of which is to take a look at something.
Fijate si dejé mis llaves en el coche.
Look and see if I left my keys in the car.
Fijate si esta lloviendo allá afuera.
Look and see if it's raining outside.
¿Te puedes fijar si dejé mi cel en tu carro?
Can you look and see if I left my cell (phone) in your car?
It can also mean to be careful.
Fijate (ten cuidado) con quien te metes.
Be careful who you get involved with.
Fijate (ten cuidado) que no te vayas a caer porque el piso está mojado.
Be careful not to fall because the floor is wet.
It can also mean to pay attention:
Look carefully or pay attention
Fíjate en lo que haces
Pay attention to what you're doing.
These are by no means the only uses of fijarse, but certainly are very common. If you've never noticed fijarse before, if you "fijate bien", you'll probably start to notice it everywhere.
¡Hasta la proxima!