Lets
you merge vertices to create a shared vertex between them by dragging 
from
a source vertex to a target vertex. Vertices can only be merged
if they belong to the same mesh. The Merge Vertex Tool options
determine the merge behavior.
Edit Mesh > Merge
Vertex Tool > 
- Target Vertex
-
The
target vertex becomes the new vertex and the source vertex is deleted.
This is the default setting.
- Center
-
A new vertex is created
equidistant between the source and target vertices. The source and
target vertices are then removed.
