Like in many previous ads we'd showcased, this one advertises a product meant to increase one's weight... a concept that seems utterly ridiculous now (except in hardcore bodybuilding circles, but seriously, those products just end up making fat people even fatter, with muscles or not).
This particular ad, according to Wikipedia, went on an interesting journey:
In 1972 Weider and his brother Ben found themselves a target of an investigation led by U.S. Postal inspectors. The investigation involved the claims regarding their nutritional supplement Weider Formula No. 7. The product was a weight gainer that featured a young Arnold Schwarzenegger on the label. The actual claim centered on consumers' being able to "gain a pound per day" in mass. Following an appeal wherein Schwarzenegger testified, Weider was forced to alter his marketing and claims.This ad doesn't feature a young Arnold (looking closely the boy's name is Walter Leno), but our previous showcase does. I have yet to find an ad that put the two of them together.
From Amazing Spider-Man #87, August 1970