It is the amount of time (ie. warning) an employer should give an employee when informing them of the termination of their employment. It also works in reverse, ie. the amount of time an employee should give the employer between declaring that they will leave their job, and leaving their job. This is usually a period of two to four weeks weeks.
Of course, there are circumstances where the employment could be finished or terminated immediately (eg. criminal act), which are usually outlined in the employment contract.