Help! This is probably a really dumb question....but someone gave me a bottle of tequila from Mexico, and it has a screw cap....but under that there is a plastic cork thing that I have never seem before, and can't open. How do I get it open???

Not sure.

I think my next move would be a corkscrew - especially if the screwcap can go back on the bottle.

I hope it's not an emergency

Enjoy Responsibly,

TTG

Is it hard plastic?

This is likely a slow pour top. Some tequilas have these its good for bars and makes it impossible for someone to add to the bottle.
Sometimes they get stuck,turn the bottle over the glass and give it a shake or a couple taps on the bottom. This should get the juices flowing!