Service Mesh is the communication layer in a microservice setup. All requests, to and from each of the services go through the mesh. Also known as an infrastructure layer in a microservices setup, the service mesh makes communication between services reliable and secure.
Each service has its own proxy service (sidecars) and all the proxy services together form the service mesh. The side cars handle communication between services, which means all the traffic goes through the mesh and this transparent layer can now control how services interact.
Source: thoughtworks.com