The pricelist.json is a file that will be created after you used the !add command for the first time and it's stored inside your files folder located at ~/tf2autobot/files/steam_account_name/pricelist.json.
It contains all the items you added via the !add command and stores the parameters you set (for example if you want to buy, sell or bank a specific item, which price you set or how many you want to buy).
It is highly recommended that you take regular backups of this file in case you will ever lose access to your machine or your file gets corrupted.
Continue: Item identifying parameters