What does the "Null" option mean in the Explore filter on my dashboard?
The Null option represents the absence of a property. For example, if you filter by Ticket group and select Null, you see all the tickets that are not associated with any group.
The Null option is by default a part of every filter you add to a dashboard. This does not necessarily mean it will be applicable to every filter. Sometimes it is not useful and does not apply to the filter at all. For example, in the Talk Default Dashboard, when you try and filter by Call Talk number the Null option appears, even though all Talk calls will naturally be associated with a Talk number.
Whether the Null option is relevant or not depends entirely on what that filter is filtering. Consult the metrics and attributes references to find what the filter is based on and how it is calculated.