A relative neologism, it considers institutional aspects of the economy not captured in the more usual micro- and macro-economy analyses.
It would include, for example, industry-level studies.
It also considers the linkages, formal or informal, between agents - whether persons or social constructs (family, tribes, partnerships, markets, civil or social institutions, etc.). It also looks at agents, for example banks or financial institutions that may otherwise be disregarded, but have actual agency themselves.
In this way it may help consideration of how theories developed for the micro level may influences the macro levels of the economy.
Sometimes written as meso-economics.
The meso part of the term means a level in between micro- and macro-economics.