pagelines_welcome_billboard can be used to modify the content at the top of the theme Welcome page produced by the get_welcome_billboard function.
NB: my_pagelines_welcome_billboard is used only as an example, a more appropriate function name should be used in production code.
The default look of the theme Welcome page is very similar to the following:
If you wanted to remove that altogether then you could simply add the following to your (Child-Theme's) functions.php file:
This would then transform the top of the theme Welcome page into something like this: