Basically a GEDCOM is a computer file of your family tree. It shows your ancestors or descendants by level - such as parents, grandparents, great-grandparents; or children, grandchildren, etc. It can also include siblings, spouses, aunts, uncles, nieces, nephews, cousins, etc..
A GEDCOM program generally will allow you to enter data (such as births, marriages, deaths), add photos, and document sources.
If you create a GEDCOM, you can then upload it to various genealogical websites, such as WikiTree!