It's a bigger Excel.

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.)