In a very simple language, a Serverless.yml file is used to code out the template according to the resources we want to create in our AWS account ... Here in this code, we are defining things like the version of NodeJs we want to use in our AWS environment ... WebsocketsApiRouteSelectionExpression: This is the custom route selection expression, meaning if we want to emit custom events from our WebSocket client, we will be passing the event name in the action property of the payload.