I agree with Elaine you should check out the GEDCOMpare help page. And to explain the basics a bit futher:
After you upload a GEDCOM file, WikiTree will generate a GEDCOMpare report that allows you to compare the person information in your GEDCOM file with person profiles that already exist in the WikiTree global tree. You can view your GEDCOMpare reports under My WikiTree > GEDCOMs.
Using the GEDCOMpare report, if you confirm a match to an existing profile, you can use data from your GEDCOM to edit that WikiTree profile (use this cautiously if the profile is managed by another member or if it looks like a fair amount of work has went into the profile).
You can also use your GEDCOMpare report to add new profiles to WikiTree, but only after you have confirmed or rejected ALL the suggested matches for every person in the GEDCOMpare report. The "add" button will only appear in the GEDCOMpare screen after you have done this. This is to ensure no duplicates are created in WikiTree when adding from your GEDCOM file.