That's like asking what Microsoft does. SAP has lots of different products, many from acquisitions, but the core product (what people think of when they say "SAP", although it's actually called S/4 HANA) is an ERP system. The Wikipedia article looks like a decent overview from what I can tell: https://en.wikipedia.org/wiki/Enterprise_resource_planning
Disclosure: I work at SAP, but on internal IT and not on any of the customer-visible products. (I don't know much about how S/4 HANA really works, except that I use it to view my payslip, enter my vacation days and such.)
