This is when your breads or cakes loose all the air they have built up to rise. It leaves a rather unattractive divot in the middle. To me it looks like a huge crater.
The best way to avoid this is to avoid jumping, and bumping, around or near the oven while cooking; taking the toothpick test too early; and opening the oven to check on your food.
Tbsp is short for tablespoon.
The Toothpick Test:
This is when you take a toothpick and insert it into whatever you have in the oven (while it is in the oven). If it comes out clean then it is done. If it comes out with dough sticking too it then it needs to bake longer.
Be careful not to check to early in the baking process; because this can sometimes cause cakes and breads to fall.
tsp is short for teaspoon.