I agree with joglekar. I think we should encourage them to read basic concepts first by themselves. rather than posting silly questions which doesnt need any hardcore knowledge but just the common sense.
Anyways, harry singh. (Still i would ecourage you to sit and read a book on STA)
Here is the explanation : Just think about it, in another way. I hope you must be a cricket fan right. So, Simply think it that way. Many times it happen that while playing a shot a batsman end up in giving loose edges which sometimes creates problem.(It usually hap with everybody). So that bowler who bowled the ball (is the arrival time) and the bastman (in order to play the shot perfectly needs to react at the stipulated time), Which is called (Required time). So if he(The bastman) fails to do so end up in giving loose edges. Thus, the difference in these times which made him give the loose edge is called The "SLACK".
It is a generic term dude. Why dont you grap a dictionary and check the meanings before posting such questions here.
cheers.