GIS stands for Geographic Information Systems and is a tool that allows data scientists to create custom maps and perform geospatial analyses to help make smarter decisions. It is a constantly-evolving blend of data, technology, analytical tools, and expertise. GIS can be used in a multitude of ways to examine patterns across geography and time. Examples of when you might use GIS include modeling the spread of forest fires, where to build a new health care facility in a community, and where to choose a study site for specimen sampling. The possibilities are limitless and the capacity to innovate with GIS technologies is always growing.
See this "What Is GIS?" video by Esri for a brief explanation on GIS:
Also, see this story map entitled "An Introduction to GIS" by Richard Harris from the School of Geographical Sciences, University of Bristol:
Starting your GIS project and not sure where to begin? See the following resources to help kick off your project (these guides can be applicable to many disciplines):