what is the physical meaning of a slack

does anybody there guys knows what is the physical meanings of a slack
does negative slack something good ?? ( me i have a feelings which say its not !!!)
does too much slack is something good ?? (me i think its a weakness in planning like taking so much factor of safety!!!)

can any body give a professional answer for me?
and if i have too much positive slack can how can i optimize it?

I will wait for some answers please