Can someone explain me Region3 please?

Wiki article: http://wiki.roblox.com/index.php?title=API:Region3

Basically, a Region3 is composed of two Vector3 points that form some kind of box in space.

You can use it for many things. Let's say you can detect whether or not a player is in a region, you could give them a pop-up gui, damage them, whatever, anything. There's even been people making invisible walls with no bricks using Region3s.

The wiki article can tell you more.

By the way next time try giving more detailed questions, more precise and with some code you tried. Just asking "what is something" or "i dont understand this" isn't the right way of writing a question, read the guide for more info. But I'm nice so enjoy this answer ;p

If it helped then no prob