Expression 'what's going down?' is an informal way of asking 'what is happening?' or 'what is going on?'.  This is a predominantly American expression. In the past tense, you might ask 'what went down?' so you might ask 'what went down at the meeting yesterday?' or 'let me tell you what went down in the bar last night'. This is a casual expression but you can hear it in both social and professional environments.

1. I think that match will go down in the record books as the best of all time!

1. I wasn't there so I have no idea what went down at the meeting. Mary was, though. You should ask her. 
2. My presentation obviously didn't go down too well with the investors. They all seemed bored and disinterested.
3. Every time the internet goes down I lose all my work! I really need to get a new router.