A Brief Look at Second Life Mesh Support is a short explanation of why Second Lifers are so excited about Mesh.
Up to now we've had Prims, polyhedrons which can be manipulated using an inworld editor, and 'sculpts', weird single prim objects that are created offworld. Now, finally, we can import 'mesh' objects of the kind used by other computer generated worlds. There are plenty of free editors out there and a common file format, now they have been thrown into the SL sandbox. This is going to be fun.
There is a new Mesh section in the SL Destination Guide that will no doubt be filling up fast.