Being on deck only guarantees the batter will get a chance to bat in the inning if there are fewer than two outs, and the number of outs plus the number of baserunners adds up to fewer than three because a double or triple play could occur. Additionally, the manager reserves the right to pull the on-deck hitter for a substitute at his discretion.
The player next in line to bat behind the on deck batter is referred to as being in-the-hole.
Significance in save situations
There are two on-deck circles in the field, one for each team, either circle can be used by either team, positioned in foul ground between home plate and the respective teams' benches. The on-deck circle is where the next scheduled batter, or "on-deck" batter, warms up while waiting for the current batter to finish his turn. The on-deck circle is either an area composed of bare dirt; a plain circle painted onto artificial turf; or often, especially at the professional level, made from artificial material, with a team or league logo painted onto it.
According to Major League Baseball rules, there are two on-deck circles (one near each team's dugout). Each circle is 5 feet in diameter, and the centers of the circles are 74 feet apart. A straight line drawn between the centers of the two on-deck circles should pass 10 feet behind home plate.
Both on-deck and in-the-hole were originally naval aircraft carrier terms. In-the-hole is the designated spot where the pilot would wait before going "on the flight deck". For safety and to avoid confusion this "hole" was below the level of the flight deck. On older carriers, it may have been an actual pit or just a designated off-the-flight-deck spot the pilot would await. So the sequence of a pilot was to be "in-the-hole" until the plane was boarded on the flight deck ("on-deck"). Then the plane taxied to the catapult where it was launched.
Media related to On-deck circles at Wikimedia Commons