Overview
Market makers are participants in the markets who trade in order to provide market liquidity. That is, the market maker maintains an inventory of the asset they are trading, and they are willing buy from other traders, and to sell from their inventory to buyers.
Market makers do not typically expect to make money by speculating on the direction of the market, rather they profit from the bid-ask spread, that is, the difference at which they buy and sell.
Inventory Models
Inventory models are meant to quantify the risk that the market maker faces by maintaining an inventory of the asset they trade, and the bid ask spread they need to maintain in order to justify the risk.