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.
Related topics
Edit Mesh > Merge Vertex Tool >
The target vertex becomes the new vertex and the source vertex is deleted. This is the default setting.
A new vertex is created equidistant between the source and target vertices. The source and target vertices are then removed.