Co-managed IT services (or hybrid IT services) are when you use a managed service provider (MSP), to supplement your in-house IT staff.  Larger businesses that can maintain an internal IT team often find that co-managed services are the best of both worlds: You decide what tasks to keep in-house and how to best leverage the MSP resources.
Here are a few examples of how hybrid IT services could work:
Your in-house IT team runs your help desk and handles day-to-day maintenance, but leans on the expertise of your MSP for cyber security, as well as general advice and assistance. Your MSP acts as your CIO (sometimes called virtual chief information security officer or VCISO services). Your in house team executes plans and manages your systems. Your in-house IT lead handles the strategy but directs an outsourced managed IT team to execute plans and support the larger team.