The practice whereby an employee who is leaving a job is instructed to stay away from work during the notice period, to spend their time at home mooching in the garden while still being paid
It is used to prevent employees from taking the piss and company information when they leave, especially when they are leaving to join a competitor.
"Man, Billy has just spat his dummy out and ditched his job"
"Put that douche bag on garden leave before he poaches all our customers"
"Man, what a douche"