What is 4wd lock compared to 4wd low?

I just bought a 2007 Dodge Ram 1500 and went to Reno, it started snowing really bad and I kept sliding so I had 4wd low and 4wd lock, I didnt know what 4wd lock was so I used the low, it was really weird if I did 40 i was at 3500 RPMs usually i'm at 1200 at that speen, so what is 4wd lock is that basically 4wd high?