A Managed Service Provider (MSP) is a firm hired to run another company's IT. Instead of every business hiring its own sysadmin, they pay an MSP a monthly fee to keep everything working.
You will typically support 10–50 client companies at once. Each has its own users, network, applications, and quirks. The job is part technician, part detective, part customer-service rep.
Two key terms: an MSA (Master Service Agreement) defines the relationship; an SLA (Service Level Agreement) defines how fast you must respond. Knowing the SLA tells you how to triage.