what does this "backboardd" mean?

backboardd is a system daemon. Among other things, it's responsible for compositing what's on screen. It has the role of the "render server" on iOS in the event-handling and rendering pipeline described in Understanding user interface responsiveness.

The fact that it shows up here is not necessarily a bad thing. It just means it does some graphics work, which is its job. You need to look at further data to determine whether the work it does takes too long. If it does, this may indicate a problem in your app, as the render server often does work on behalf of your app.

If you determine it is not indicating a problem in your app's rendering pipeline but it instead shows up incorrectly here and seeing backboardd is misleading, please file a feedback.

If you think that specific documentation about backboardd or the instruments you are using would be helpful, please file a feedback as well, describing what information you are looking for.