What does welded and machined mean?
A rim is a piece of aluminum extrusion, rolled and joined into a hoop. 

There are several types of joints used. 

Rims can be pinned together using a piece of metal at the seam to hold the alignment.

Or the seam can be aligned and welded. This is the strongest method. Welded rims used to have a small bump at the seam from the welding process, which would wear smooth from brake use. 

Someone came up with the idea to weld the seam for strength and then machine the braking surface smooth, which is the strongest and best kind of joint possible on a rim.