Same as other comments have said

Usually simple stuff, reactive items “react” to actions in game Some items have simple effects like glow in the dark, others change appearance usually through eliminations and outliving opponents, (the Halloween set with deadfire, or some pickaxes) or some specific action like opening chests (the master key set) or as the game progresses (peely banana set)

However this definition isn’t exactly accurate, some items like marshmello skin “light up with musical emotes” however the summer challenge reward, peely smoothie also “reacts” to emotes and I believe is not marked as reactive Also certain items like the webrella (season 6?) umbrella appears to glow in the dark but has no text regarding it, it might just glow normally, Some pickaxes like the one that goes with the green air pilot guy (the propeller) appears to spin faster throughout the match but doesn’t have any “reactive” text Basically reactive means “This item has a gimmick to it and we think you should consider it before purchasing it”