What does TR mean on an electrical outlet?

TR on an electrical outlet stands for "tamper-resistant." Tamper-resistant outlets are designed with spring-loaded shutters that cover the openings for the plug blades when the outlet is not in use. These shutters help prevent accidental contact with electrical contacts by children or anyone else who might be tampering with the outlet, reducing the risk of electrical shock or electrocution. The shutters only open when equal pressure is applied to both sides of the shutters simultaneously, as would happen when inserting a plug. TR outlets are now required by the National Electrical Code in certain areas of residential construction, including in new homes and areas where children are likely to be present.