A plug weld is created by drilling a round hole through one of two overlapping members, then filling the hole with weld metal that fuses to the lower member. The result is a strong shear-resistant joint without welding around the perimeter of the upper member.
Plug welds are common where you need to fix a stiffener plate to a face without a visible perimeter weld, where access for a perimeter weld is restricted, or where the upper member is too thin to weld at its edge without burning through.
The slot weld is a related variant: instead of a round hole, a slot is cut and filled with weld metal. Slot welds give more shear capacity than a single plug for the same amount of weld metal.