.... as set at end of NR

I am sure this has been asked b4, i am brand new to knitting. Shapeing the raglan armholes, it says
Dec one st as set at each end of NR then every foll 4th row…
Not sure what the as set means.

Welcome to the forum!
“As set” refers to an earlier row where the placement of the decreases and probably the type of decrease are specified. See if you can locate that row just a couple of rows before the “as set” direction.
What is the name of your pattern and designer?