According to Wikipedia,
Market microstructure is a branch of finance concerned with the details of how exchange occurs in markets… The major thrust of market microstructure research examines the ways in which the working processes of a market affects determinants of transaction costs, prices, quotes, volume, and trading behavior. Recent innovations have allowed an expansion into the study of the impact of market microstructure on the incidence of market abuse, such as insider trading, market manipulation and broker-client conflict.
It’s an area to look at to understand how the market works, and can be used not only in high frequency trading but also in many areas and strategies building.
Below are few (recent) open-access resources and academic research & applications on Market Microstructure topic: