The word stands for GEnealogical Data COMmunication. It was a file format developed to standardize the way genealogical data could be transferred from one computer program to another. If someone keeps their family history data on their computer in a program like Roots Magic, Legacy Family Tree, Family Tree Maker or Personal Ancestral File, even though the programs are made by different companies, they all offer the ability to both import and export your information using the standardized GEDCOM format.
If you keep your family history on your computer in one of these programs, you can follow your program's instructions to "Export" your information in a GEDCOM format. Then you can upload that GEDCOM file to WikiTree, as an alternative to typing in each name individually.
You don't have to use a GEDCOM, but if you've already spent time typing hundreds of names into your desktop genealogy program, it could save you time rather than retyping each name individually again into your WikiTree. But that's not a problem either.