The phrase "second set" usually refers to the second part of something, like a performance or a game. It comes after the first set and usually follows a break or intermission. Full definition
This allows a potentially second set of eyes the chance to see some examples of what you've done in the past to mitigate pain. (ivyexec.com)
For example, if you go from doing 6 reps for the first set to doing 2 or 3 reps for second set, then you probably need more rest between sets. (kingofthegym.com)
Excellent reliability,, 3 set tire just put on second set brake pads. (kbb.com)