What are stainless steel slip-on flanges?

Answer 1

The link below will tell you all you need to know about flanges

Source(s): https://www.texasflange.com/

Answer 2

a flange is a sleeve of sorts that is used to connect two distinct pipes or ducts or mass transfer conduits of whatever form you want to imagine.  The general idea is that each side has a matching flange and the two separate flanges can be attached together (usually bolted) to provide the necessary linkage between two different conduits.  One of the basic forms of a connector.

Slip-on means that you can slide the pipe end of it right onto the conduit (does not screw on).  Stainless steel is what the flanges are made from, their composition.