No problem with questions! The crop is the organ where a chicken's food goes to be ground up for easier digestion. This article has a great diagram of where the crop is:
https://www.theselfsufficienthomeac...-sour-crop-and-impacted-crop-in-chickens.html
When your girls eat a bunch it will get visibly bigger and rounder, and if you gently feel their chest you'll be able to feel it in there, kind of like a round ball beneath the skin. In the evening it should be full, but first thing in the morning you normally won't be able to find it as it SHOULD be empty from their overnight fast. If it is still round in the morning before they eat, there's likely an issue with the crop.
Edit: You can check for lice and mites by parting the feathers and looking for little bugs or eggs (color may vary) near the skin or on the bases of the feathers. Popular spots are the vent and underwings I believe, but there's probably an article or other post you can find with the search function for doing a really good examination. In my experience, the vent is where I've spotted them the most.