Data integration can mean a lot of things to different organizations.  At one end of the spectrum would be approaches like manually importing/exporting spreadsheets to copy data between systems.  At the other would be systems that are tightly connected, for example via APIs, so that actions occurring in one system are almost instantly passed to the other system to handle whatever related tasks it needs to do.  This level of integration can make multiple systems effectively behave as if they were part of a single, overarching process for running the business.