A custom backend web service is a separate application that runs on a server, that communicates via the network.
It communicates with the web browser and/or other services.
This is also sometimes called "custom API development", and often accompanied by marketing hullabaloo, but it is basically the same thing.
From the perspective of an existing web application, a web service is a tool that provides a method of augmenting the application with additional capability that is not available in its environment. It is a way to add new data or features, or to integrate several systems together to share data.
Reusabit develops custom web services to breath new life into existing systems. If you feel like your existing solution almost works, but not quite, give us a call, we can help find the right solution. There is a good chance that that solution might involve a custom web service.