A microsite is a smaller, simpler website used to promote a specific subject, product, or campaign. Often, these sites will be connected to your company in some way (such as by having a project page on the main site link to a microsite about that project), but will be hosted separately on a different domain, giving them a level of separation from your primary brand and content.
Generally speaking, the point of creating a microsite is to keep users hyper-focused on the content published on the microsite, making them synonymous with landing pages in many ways (both strategically and in their implementation from a design and technical perspective).
For example, during the merger and acquisition process, the combining firms could create a microsite to help coordinate information presented to clients in a centralized location.
Other common use cases include business development tools (such as small website devoted solely to “get a quote” forms, proposals, or other similar marketing resources), event-based sites (such as for an upcoming sales meeting, conference, or other promotional event), and DEI or other resource pages for specific initiatives in your organization.