It refers to the practice of police questioning or arresting a Black homeowner in his own house for no apparent reason.
Henry Louis Gates Jr., the pre-eminent African-American studies scholar at Harvard, was arrested for Homeowner While Black after a neighbor reported of two Black men breaking into his home. Mr. Gates had just arrived back from a trip to China and found his front door jammed. He entered through the back door. When the police arrived he was questioned, produced ID to prove he lived there, and was arrested.