Programming

From an Academic background SAP was hardly ever mentioned and I have hardly any idea what it does. I am studying cs and have a practical coming up in the Future and many companies I have looked at state that they are working with SAP. Now what does that mean ? Is it just a set of libraries or APIs in use or how does it influence you from a Developer standpoint?

This question is probably a little vague and slightly off topic but from what i've read on the internet sap seems to be everywhere but theres no good explanation of what it encompasses, which is why i am asking (I have literally no idea) Appreciating any answers!

Edit: i should probably say that im not talking about companies receiving service in form of applications by sap but stating they develop extensions of some form