The Lab is an expansion to the Ranch that houses a barn containing Hobson's old, Slime Science workshop. Once unlocked, the Lab will allow you to fabricate dozens of useful gadgets using Slime Science!
The Lab is a patch of land with some metal crates and mechanisms thrown around, with a large barn found in the center. Inside is the Slime Science workshop. To know more about it, go to this page. It is connected to the Ranch, and to the Grotto.