Ask HN: What is LLVM (from the perspective of a middling Rails developer)?

I generally understand that a compiler takes code (written in a language that needs to be compiled like C) and generates machine/assembly(?) code from it -- but I don't really understand why that makes LLVM the big deal it currently is. I don't have a CS degree, but I understand some of the concepts (like notation, sets, etc).

Can someone ELIaRailsDeveloper what the LLVM and why that is a big deal?