After opening Metadata for a project (the project has no metadata yet) you will see something like the following.
Next change the selection "Default Metadata Screen" (it is on the top right of the page) to "Software Project Metadata Screen".
You will notice that the page is no longer empty.
In the "Software Project Metadata Screen" it was specified that you should specify a "Management Summary" or the "Start Date" or ... for the given project.
A Metadata Screen allows Jira-Administrators to specify which metadata should be filled in for a project. They also have the option to suggest values to you:
This should simplify the cross-project search for issues based on metadata (How-to search for issues related to metadata values).
Besides the existing screens you can define your own or change the default screen. You can delete existing screens, except if the screen is set as default.
Futher details: How to customize the Metadata Project Screen as Jira-Administrator