Multirepo is a software architecture which means a separate repository for each project. Each project is an independent working unit. It has minimum tooling investment and is in use at companies such as Amazon and Netflix.
You can think of microservices as a software design architecture where the aim is to keep maximum parts of code independent/ loosely coupled.
On the other hand, monorepo and multirepo can be seen as a code management architecture.
A software following the microservices architecture can be implemented/ managed as a monorepo as well as multirepo. Let me know if this clears the difference