Preclear means someone who has not yet become clear. The Voting Rights Act requires some jurisdictions to receive advance approval from the Department of Justice for changes they want to make in adjusting voting districts or procedures.

Under the preclearance provision of Voting Rights Act, jurisdictions with a history of discriminating against minority voters must receive advance federal approval for any changes in voting practices or procedures. 

Now that the Robert's Court removed preclearance of the Voting Rights Act, the only other legal reference point is in regard to border crossings.

According to 19 CFR 122.1 [Title 19 Customs Duties; Chapter I U.S. Customs and Border Protection, Department of Homeland Security; Department of the Treasury; Part 122 Air Commerce Regulations; Subpart A General Definitions and Provisions], Preclearance is the examination and inspection of air travelers and their baggage, at the request of an airline, at foreign places where Customs personnel are stationed for that purpose. Preclearance may be used only for air travelers and their baggage, not for merchandise.