In the general sense of the phrase it is exactly as Dustin described it, but in the narrower usage it refers to an underlever type that required no manual movement, but was bolted by the spring action of the underlever. This was deemed by many to be an improvement over the Jones type underlever which required the lever to be manually moved rearward, bolting the action without the benefit of any automatic (spring assisted) mechanism. So, in the narrowest usage it is "as opposed to" the Jones underlever. Maybe that is what the OP was referring to. (?)
See "Snap action" in Hallowell's excellent firearms dictionary for a description and diagrams:
https://www.hallowellco.com/abbrevia.htm#S
