What does over-proofed dough mean?

The answer

Over-proofed means the dough has fermented too long, so the gluten has overstretched and the yeast has used up much of its food. Signs are a dough that looks very puffy but feels fragile, a floured poke dent that stays down instead of filling back slowly, and a loaf that collapses when scored. The baked bread is flat with a coarse crumb and a sour or sharply yeasty taste. A slightly over-proofed dough can sometimes be reshaped for a shorter second rise.