Good question, and a good thing to know for new chicken owners.
Impacted crop is a term used to describe the condition of a bird who had eaten something that had lodged in it's crop. Normally they have rocks and hard objects tumbling around in the crop to grind things. Sometimes they eat something that can't be ground, or just gets wedged in there. This causes food-backup, and yeast or bacterial infections.
you can tell a chicken has this because their breath turns bad smelling on an unholy level, and the crop often bulges noticeably (and never goes flat like it does normall).