When you publish a service you can specify how many processes (min / max) your service can use. As requests are being served memory is allocated, state is changed, some processes could go corrupt
Recycling is the process destroying and re-spawning processes that are running to maintain a good memory footprint.
Here is a good read on recycling
http://enterprise.arcgis.com/en/server/latest/publish-services/windows/tuning-and-configuring-services.htm
Enjoy
Hussein
The Backend Engineering Show with Hussein Nasser