Original Jurisdiction means which court has the right and obligation to hear a case first. A Court with original jurisdiction tries the cases and builds out a factual record.
In most cases, the Federal District Court is the court of original jurisdiction.
The upper level Courts, including the Supreme Court, are normally appellate courts. That is, they review the record from the court of original jurisdiction for clear error, etc.
In some cases, like matters involving ministers or ambassadors, the Supreme Court is the court of original jurisdiction.