Microservices is an architecture pattern where applications are built as independent, loosely coupled services. Each service handles one business function. A user service handles user authentication and profiles. A payment service handles transactions. An order service handles orders. These services are separate, often with separate databases. They communicate through APIs.
This is different from monolithic architecture where everything is one codebase and one database. Microservices scale well. If the payment service gets high traffic, you scale that service without scaling everything else. Each service can be deployed independently. Fix a bug in the user service, deploy just that service. Microservices enable large teams to work independently.
Different teams own different services. No one team needs to understand the entire system. The downsides are operational complexity. Managing many services is harder than managing one. Distributed systems are harder to debug. Network calls between services are slower and less reliable than function calls. Data consistency becomes harder. Each service has its own database.
Keeping them in sync is complex. Microservices work best at scale, when you have enough traffic and complexity to justify the operational overhead. A startup with one small application shouldn't use microservices.