A good geographer is able to handle data in a variety of situations and a variety of ways. Geographical data handling includes the collection, interpretation, analysis, presentation, application and evaluation of both primary and secondary data. That sounds like a lot of complicated stuff to do, but if you take it step by step, it becomes a lot easier.
Primary data is original data that you collect for yourself during fieldwork, it is likely that no-one else has collected the exact same data before. Secondary data is data that has already been collected by someone else, usually for a different reason, and is gathered from GIS material and other digital sources, graphs, maps, charts, diagrams etc. Data can take many different forms - numerical, statistical, the results of questions you have asked and so on.