Undercutting is when the edge of a weld has an inverse U shape. This is a weakness in the weld that typically is caused by when the travel speed is too high. Then the weld bead will be very peaked because of its extremely fast solidification. It also can be caused by too high amperage.
Picture it this way, the forces of surface tension have drawn the molten metal along the edges of the weld bead.
The undercut groove is where melted base material has been drawn into the weld and not allowed to wet back properly because of the rapid solidification.
Decreasing the arc travel speed will gradually reduce the size of the undercut and eventually eliminate it.
When only small or intermittent undercuts are present, raising the arc voltage or using a leading torch angle are also corrective actions.
Acceptance criteria specify in different specifications and services such ASME B31.1, ASME B31.3, AWS D1.1, AWS D1.6, API 1106, ISO 5817, ISO 6520-1, etc.