The Vertex Drag Weld tool allows you to interactively weld vertices together. To use, simply LMB-click drag on any vertex on your geometry and and move it toward the vertex you wish to weld it to, it will snap to the vertex when it gets close. Releasing the mouse button will weld the 2 overlapping vertices together. When enabled, users can also RMB-click drag in the viewport away from any geometry to interactively adjust the falloff. For more on individual tool settings see move, element falloff and geometery snap.
