RTH (Regular Trading Hours)
Nowadays, many markets trade 24 hours a day. That was not the case before this Internet age and most businesses/trading firms still operate within normal day time hours based on the time zone they are located at. Most of the active traders still need to sleep. So exchanges maintain 2 sessions (or more) for those instruments that are traded around the clock. The Regular Trading Hours is the session when normal businesses are operating at the location. The After Hours is the session outside this schedule.