Six-pointer is a term used in association football circles to describe a certain kind of very important match, in which not only the points gained from a win are important, but also the points lost by the opponents.
Success in a league system depends, of course, not only on the results of your own team, but of those around it. Winning a game is all very well, and will get you your three points, but if all the teams above you win as well then you are unlikely to move up the ladder.†
However, lets look at the example of the perfect six-pointer; a game in which your opponents are themselves only two points above you in the league. A win would guarantee moving up the table. This of course, also applies to a game where a defeat would be disastrous for the same reasons.
Bear in mind that, especially towards the end of the season, six-pointers do not have to involve teams strictly within three points of each other but this is the general usage.
† Unless, of course, you were previously level on points and could move up or down on goal difference.