It indicates the mileage before the next service is due.
If it reads '0' then the service is due now. If it reads with a minus '-' then it is overdue by the mileage shown.
Note: this is easy to reset (no tools required) and is usually done by the garage mechanic after a service but can easily be done by a competent DIY-er.
Teh spanner symbol means it's service time.
Service due
that the car is due for a service
Its due a Service , or there is a problem
it's service time
This means the car is starting to need a service.
that means its due for a service
it needs a service
It is to advise the driver that the van requires a service.
It means that your car think it needs to be serviced.
it means you have a fault needs plugging into a diagnostics computer.
Becasue the car is fixed.