What is "a plug or slot weld is what type of weld?

A plug weld or slot weld falls under the category of Fillet Welds, but is a specific type used to join overlapping members. It's a fusion weld made by filling a hole (plug weld) or a slot (slot weld) in one member of a lap joint, connecting it to the surface of the other member exposed through the hole or slot. They're primarily used for lap joints.