"Viewer discretion advised" refers to "adult" content that is unsuitable for children, and for some adults as well. It means that adults should make a conscious decision whether or not to watch it, and be VERY careful about lettng children see it.

What does discretion mean?

2 : the quality of having or showing discernment or good judgment : the quality of being discreet : circumspection especially : cautious reserve in speech. 3 : ability to make responsible decisions. 4 : the result of separating or distinguishing.

What viewer means?

: one that views: such as. a : a person who watches television. b : a person legally appointed to inspect and report on property. c : an optical device used in viewing.