In essence it makes note of all songs you've played in your set (or say the last 4 if you're on event mode), and allows you to take a screenshot of it.
I'm not entirely sure of the implementation process but since we have a copy of SL it shouldn't be too hard to suss out.
This is something that Simply Love implemented that I think should be worked into GrooveNights as well: https://fbcdn-sphotos-c-a.akamaihd.net/hphotos-ak-prn2/t31.0-8/10007375_534375396670842_3394894348618798773_o.jpg
In essence it makes note of all songs you've played in your set (or say the last 4 if you're on event mode), and allows you to take a screenshot of it. I'm not entirely sure of the implementation process but since we have a copy of SL it shouldn't be too hard to suss out.