According to a February 2020 report from the GAO:
Nuclear microreactors are very small reactors usually generating less than 50 megawatts electric (MWe). They are seen as an alternative to small modular (50-300 MWe) or conventional reactors (often around 1,000 MWe). By comparison, microreactors can be produced more quickly, and within weeks, transported and deployed to locations such as isolated military bases or communities affected by natural disasters. They are designed to provide resilient, non-carbon emitting, and independent power in those environments.
The report outlines opportunities, challenges, and policy issues related to the development and deployment of microreactors.