Post-market means after the market. Some financial markets, like foreign exchange markets, never stop trading, but most individual stock markets have defined trading hours. That’s because the stock exchanges are open generally for the working day of the time zone in which they are located. Post-market generally refers to the late hours after the stock market closes in any jurisdiction (see also pre-market definition).
In some cases, company announcements like earnings reports will be released post-market. That gives traders and investors time to digest the information before the regular stock market session opens the next day.