The press, at its most basic, is a second bet that begins during the course of a round, joining and running concurrently with the original bet. When one player presses, he is starting the second bet, or "pressing the bet." The second bet is usually for the same amount as the original bet.
Players can agree to use presses with any type of match, but the Nassau is the "home" of the press, and pressing is easily most associated with Nassaus.
There are some regional differences in the meaning of "press," however, just as there for many other golf side games and betting formats. So note that in some places, a "press" or "pressing the bet" simply means to double the amount of the original bet.
See our FAQ, "What does it mean to 'press the bet'?" for much more on presses.
