High Altitude, Low Open - a method of
parachute insertion used by military personnel whose desire/need to remain undetected overrides their perfectly normal desire to avoid risk. In general, it's just what the name implies - one
leaves the aircraft at a high altitude, usually at night, and
free-falls to the minimum safe height for parachute
deployment. At that point, you pull the
D-Ring and pray, like all other jumps.
The advantages of this approach to aerial insertion are many. Your ferry aircraft can fly high enough to avoid ground fire or, even better, detection. You spend as little time as possible in the air, hanging from your highly-visible parachute, as a target. You have the ability to jump offset from your target, using your freefall time and altitude to translate to the target point (glide). Plus, it looks really cool on film, gives you maximal 'I'm a baaaaad-ass!' time in the air, and allows the Armed Forces to produce spiffy recruiting videos. Roninspoon reminds me of what may be the largest advantage - the distance that the jumpers can travel during drop means that their aircraft can sometimes remain in friendly or neutral airspace even when their landing point is, well, not so welcoming (and/or better yet, not so legal). This, 'spoon points out, is quite useful to those organizations that do covert-type stuff.
Of course, there are disadvantages as well. The high altitude means you must have more gear to survive the drop - for warmth, or if high enough, for breathing. The low opening means there is little to no margin for error - if something goes wrong, there probably is not enough time to do anything. Finally, dropping at night and/or over water to avoid detection on the ground may be effective, but increases the dangers to the jumper on landing.
For perhaps the ultimate in HALO jumping, read the opening chapter of Robert Heinlein's Starship Troopers. No, not the movie.