C4 Model
The C4 model is a set of abstractions and diagrams for visualising software architecture.
See the C4 model
System context diagram
The system context diagram is the first starting point, where you express the very top-level systems.
A "software system" doesn't mean a program. It means any number of programs which come together to form a system, that does a thing.
Example
If you have a single website that features a store and a blog, a backend to handle payments, a database to hold the blog content, and an administration portal for managing the store, then
You could argue that you have two systems, a Store and a Blog. That's all.