In a restaurant, on-call scheduling is when employees are scheduled for a shift that they may be responsible for working. Typically, the restaurant will notify on-call employees within 24 hours to let them know if they are expected to show up and work their shift.
For example, let's say a server has an on-call shift starting Friday at 4 PM, and the restaurant has a policy where on-call employees need to call between one and two hours ahead of time. This server calls her manager on Friday at 2:30 PM, when she is notified that she is indeed expected to show up for her 4 PM shift. The server then shows up to work as if it were any other scheduled shift.