A Japanese sports expression meaning, literally, more wins (than losses, obviously).
In sumo getting kachikoshi means that the sumotori (sumo wrestler) has won more than half of his bouts in a particular basho (tournament). This means 8 wins in a 15-day basho, and 4 wins in jungyo (7-day tournaments, held around Japan in even numbered months)1.
Achieving kachikoshi means getting promoted on the next banzuke2 (ranking list). The smallest step of promotion possible is a half-step from eg. maegashira 6 west, to maegashira 6 east.
For yokozuna (Grand Champions) and ozeki (Champions) different rules apply. If there are more than one yokozuna they can be shifted from east to west, and, in case there are three of four of them they can be shifted up and/or down. But they cannot be demoted from the rank of yokozuna (though they are expected to withdraw if they begin to perform badly - and yokozuna are expected to always get kachikoshi). The rules are much the same for ozeki, though they can be demoted from their rank should they suffer makekoshi (more losses than wins) in two consecutive basho.
- The results in jungyo does not affect ranking in banzuke.
- At the very least kachikoshi will ensure that the sumotori does not get demoted.
My sources are Sumoforum.net and countless hours of watching sumo.