I'll give you the web definition of it: Training conducted on-site and in-house either by staff or contracted trainer.
Basically, it's staff development.
Some places have mandatory 'in service' to ensure their staff is fully trained and staff is following proper and safe procedure --- at their employer site. I was required, in the hospital, to have 'x' number of hours of in-service training per year.
I'm not sure about relationship of inservice and certification retention. Someone else may be able to tell you if their is mandatory inservice --- I get the impression at least some states have that in order to retain your certification.