Answer:
See explanation
Step-by-step explanation:
We talking text language or science?
If you mean text, I'm afraid I've absolutely no idea. Kids nowadays have an acronym for everything because they are just too lazy to type accurately and spell it correctly XD
If we're talking science, AVM stands for Arteriovenous malformation. It's precisely what the name implies, a malformation (an abnormal/unusual/faulty) connection between your veins and arteries, and this is typically present in your spine (but also the brain). It's considerably rare as a condition.
What this could most severely result to is the case where your arteties and/or veins (but usually both) rupture and you're bleeding out internally into your spine or brain, resulting in a LOT of really not-fun problems that can be and are very life-threatening.