/click simulates a button press. The WoW APIs let mod authors create virtual buttons and attach macros to them. When you do "/click SomeSequence" in a normal macro you are simulating a L-click of the invisible button named "SomeSequence", which MacroSequence has created with the sequence macro you specified.

More info on /click here:
http://www.wowwiki.com/Making_a_macr..._button_clicks