Stim is short for "stimulants," like the amphetamines that the Air Force feeds to pilots. So no, it's not a medical thing, in fact it's bad for your troops' health. It gives them a short boost to speed and reflexes, though, so they can move faster and shoot faster. Sometimes they accidentally kill Canadians but oh well.
military's used to and in some cases still use what we now consider illegal drugs to get troops battle ready and to give them a boost in combat. i forgot the specifics but ww2 and before crack cocaine and other drugs were widely used to give the troops a boost when they needed it. then agian illegalizing drugs was something only done rather recently.
as for sc2 i couldn't get into but i could definitely get into the single player campaign and its lore was quite immerisive. just gameplay wise I don't find the multiplayer as fun as coh.