"Game Over" is a message in video games which signals that people failed a game, commonly due to a negative outcome such as losing all of one's lives - although the phrase sometimes follows its score after successful completion of a game. Extremely lazy games in the 80s-90s would sometimes display their standard game over screen as the "ending" of the game, something that was always heavily cricitized. The phrase has since seen wider use to describe the end of an event in real life.
The phrase is occasionally used to indicate the end of an argument or process in real life. In January 2011, protesters and rioters in several North African and Middle Eastern countries used the slogan "Game over" on banners to express their anti-government sentiments.
The concept of the game having a failure state that resets all progress originates in coin-op arcade games, where it would be the final message displayed after the continue screen timer had counted down to zero, with the idea being to reset the game state for the next player to come along to start afresh. This in turn was because the content in many period arcade games was heavily front-loaded with the first levels being the most impressive and the later levels often deliberately designed to be difficult and frustrating so a player would quickly empty their wallet and leave.
While there have been some claims that systems based around limited lives and a permanent fail state are in some way obsolete, this isn't really the case: it would be very hard to imagine, say, a scrolling 2D shooter that did not function based on a lives system. It is, however, true that the much greater length of many modern games renders a full reset too punishing for the average player (though there has been a recent trend towards games with hidden "permadeath" super-difficult settings that delete the player's save file on death). Many modern games that use lives-based systems also feature checkpoint saving: for example, they may let you continue at a mid-stage checkpoint until you run out of lives, then require you to continue from the start of the level instead. Kicking the player right back out to the title screen is considered poor form in most genres, however.