Resin.io is a service that makes it easier to manage IoT devices like Raspberry Pis by handling setup, deployment, and access without needing to configure devices directly. It allows cloning code from a sample repo, pushing it to deploy on a device, and accessing a web server running on the device through a provided URL, all without complex setup. The service also enables syncing code changes and SSH access via a VPN connection between devices and Resin.