They are a program which uses a database of items and prices to manage a users' items on the toribash market here. Common functionality includes automatically updating the price of items and buying/selling items based on the values of the database as well as other parameters and things the creator has added.
They saw a slight rise in popularity a few months ago when the shop API was released but have just been made illegal as they make the market unfair for users without a bot. (Since bots can operate around the clock 24/7)