A context diagram is a high-level data flow diagram. The focus of the context diagram is the system that is undergoing a change.
The context diagram shows all the internal and external relationships to the system. It shows interactions with stakeholders, other systems, and the data flowing into and out of the system.
The context diagram is very effective at providing a high-level view of what will be affected by a change to the system. It provides the high-level view or high-level requirements for the change.
For clarity, a system could be an IT system, a SaaS the business uses, or a defined process.