What does the _blazor?id= part of a Blazor Server URL mean?

I have a Blazor Server web app. When I look at the DevTools in my browser I see a lot of GET requests to URLS like https://mysite/_blazor?id=123456789&_=123456789 for one page load.

What are these GET requests doing? What is the id parameter? And what about the _?

Is this the ID of the connection? Is this SignalR doing its thing?