Grabbing a "1-up," usually appearing in the form of a green mushroom, will result in an additional life, or "man," in many Nintendo games. These green mushrooms are normally found in a "?" Themed Box scattered around the Mario Universe. In games not related to Mario, "1-ups" are not necessarily in the form of a Green Mushroom. They may take the form of any number of things that all add another life or man to the player.
The "1-up" has its origins in the arcades. Those games were made to be very difficult, and eat up the players' coins. Because of this, the "1-up" was one of the most coveted items that could be found in a video game.
In modern games, however, the relevance of the "1-up" is on the decline. Modern games, since they are played on consoles and do not require coins to continue, do not rely on "1-ups" to lengthen the game time, but simply give every player infinite chances to finish the games.
While the "1-up" is becoming an ever more rare in-game item, they shall never be completely lost, for as long as there is a single "1-up" left, we may rise and fight another day.
In the Mario & Luigi series
, the 1-Up revives your fallen bro to 50% of your health while the Deluxe version doubles the revival power to 100%.