For digital mapping questions about geographically displaying genealogical data. GIS is an abbreviation for "Geographical Information System", a digital representation of location-based information that (among other uses) can be displayed on a map. For questions about historic maps, use the "maps" tag instead.
The term (Geographical Information System) describes any system that collects, stores, edits, analyzes, integrates and displays location-based information for the purpose of supporting or otherwise improving decision-making.
GIS were originally presented as marks on sheets of paper and have evolved as more powerful and flexible technologies have become available. Modern digital GIS can be thought of as highly-specialised databases.