What is it?
Managed IT services cover several different areas and Managed Wi-Fi or Managed Network Services as it's sometimes called is just one aspect. Like all Aspects to Managed IT Services it's outsourcing to specialists that focus on one or more areas, although by calling it Managed Wi-Fi it is slightly misleading as it also includes routers, switches and everything that enables electronic communication.
It's made possible by technology called software defined networking (SDN) that allows your core infrastructure to be controlled, managed and monitored via software often remotely through secure connections.