According to BBC News, the euphemism describes the typical manner in which a summons from the MSS is presented:
"The invitation comes from the authorities in the form of a phone call, and a knock on the door. Those being invited range from celebrities who have expressed strong views on a topical issue to well-known dissidents and young people who get bold on the internet. The questioning normally lasts a few hours - tea might or might not be drunk during the session. The security people will ask you about your activities and issue warnings to stop or face the consequences."[1]
The interrogations occur primarily in police stations, although some have reported being cornered at workplaces or in schools, or at home, with police forcing their way in.[2] According to activists who have experienced the interrogations, invitations to tea don't involve beating or sustained verbal abuse as they serve as the bottom rung of the government intimidation and persecution ladder, and "depending on how big a threat you are in their perception, things can become much worse."[2] According to Reporters Without Borders, reporters and dissidents who persist often find the invitations to tea are followed by criminal proceedings or arbitrary detention in the country's black jails.[3]