Envoy Filters Query

1

hello all… i am really new to envoyfilters…
a) can someone explain me what this envoyfilter does and what happens if i apply this envoy filter ?
b) How to test its functionality like before applying this and after applying this?

apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: bms-operator-platform-service
spec:
  workloadLabels:
    app: bms-operator-platform-service
  filters:
  - listenerMatch:
      listenerType: ANY
      listenerProtocol: HTTP
    insertPosition:
      index: FIRST
    filterType: HTTP
    filterName: "envoy.grpc_web"
    filterConfig: {}