A Service Mesh is a dedicated infrastructure layer for facilitating service-to-service communications between microservices. It manages and optimizes the communication between microservices by providing features like load balancing, service discovery, encryption, observability, and retries. A service mesh decouples these functionalities from the application code, allowing developers to focus on writing business logic. Popular service mesh implementations include Istio, Linkerd, and Consul. Service meshes help improve the reliability, security, and observability of microservices architectures, making it easier to manage complex distributed systems.