They are directions indicating how many times per day the dose should be administered. It goes like this:
SID or QD = Once a day
BID = Twice a day
TID = Three times a day
QID = Four times a day
However, The Joint Commission which acredits hospitals and nursing homes has recommended that these abbreviations be discontinued because they lead to errors in dosing. Now, physicians must write out the words, "once a day," "twice a day" and so on.