What is slack?
'Slack' is the amount of time you have which is measured through while an event ‘really happens' and while it ‘should happen’.
The term ‘really happens' can also be considered as being a predicted time for while the event will ‘really happen'. While something 'should happen' can also be termed as a 'deadline' so another definition of slack would be the time from while something 'really happens' (call it is Tact) till the deadline (call that Tdead).
Slack = Tdead - Tact.
Negative slack means that the 'reallyhappen' time is later than the 'deadline' time...conversely it's too late and a timing violation....you have a timing problem which requirements some attention.