Envoy is a network service proxy which sits alongside your applications and does the heavy lifting on the network. Instead of having to implement custom service discovery, retry logic, load balancing, circuit breaking in your application, you can let Envoy Proxy handle it all.
Envoy is generally paired with Istio in a Kubernetes environment. This talk goes into how Monzo uses Envoy Proxy with a custom configuration/discovery service and how it’s implemented. You don’t need to bring the heavyweight Istio just to use Envoy!