The verb "meterse" has many uses, one of which is "to get in". For example:
Me gusta ir a la piscina, pero no me meto
I like to go to the pool, but I don't get in.
If you're like me, that usage alone was an amazing discovery for me. But it turns out there's another usage as well.
No te metas conmigo, que no me conoces
Don't mess with me, because you don't know me.
And yes, that can carry a warning or threating like tone.
Here's a few more examples:
No me meto con qualquiera
I don't get involved with just anybody
No te metas con mi hermanita
Don't mess with my little sister
No te metas con drugas
Don't get involved with drugs
No te metas en mi vida
Don't interfere in my life
This pretty straight forward, and you should be able to start using this right away, and if you haven't really noticed it's usage in Spanish before, I imagine you'll start to see it everywhere now.
¡Hasta la próxima!