What is it?
Tree merging will allow you to find other trees with ancestors in common and combine your tree with theirs.
It basically give the means to join currently separate Geni trees into a larger tree. (It is a prerequisite for GEDCOM import feature, which makes it possible to load already created family tree data into Geni.)
In its initial release, you'll be able to merge two unclaimed profiles, or else a claimed profile and an unclaimed profile. (claimed vs. claimed requires an additional email flow that obtains authorization from both email addresses; Geni is saving that for a later release so that they can get 90% of the functionality into your hands sooner). So Geni will choose which one to keep based first on whether one is claimed; if not, then the more-complete profile (per Geni's profile completeness bar) will be chosen. In the event of a tie (likely for two new profiles), the user's input will take precedence (did you drop A onto B, or B onto A?)
Once the profiles are merged, the secondary profile will be accessible from the profile page of the primary but none of the profile data will be merged yet. The merge can be undone at any time until the user clicks the "Automerge" link on the secondary profile, at which time Geni will automatically merge whatever we can (initially photos, guest book comments, email addresses, and possibly more) and the merge is considered final. The secondary profile will remain, with only the data that could not be automerged, until it's deleted using a link or button on the secondary profile page.
Later releases will refine these features and implement more complex ones.