Overblowing is producing a different note in a wind instrument by forcing air harder. This can be a deliberate technique to get additional notes, or an inadvertant problem which results in notes other than those intended.
Overblowing with bagpipes
With bagpipes, overblowing is a problem that arises when playing the bagpipes. A phenomenon perhaps unique to piping, it is the most common reason for unsteady tone.
When a piper plays, a rhythm is set up between blowing into the blowstick and squeezing the bag. Often, a piper will oversqueeze the bag while still exhaling, causing the pipe to vary its tone. This problem can be fixed in a number of ways:
1) Using a slightly harder reed, or modifying the existing one to take more air.
2) Practicing with a manometer to increase steadiness.