It means a person who studies baseball statistics. It’s not a particularly well-used word anywhere, though better known than it was, but it’s almost unknown outside North America, even among the minority who follow baseball in other countries.
The game, I am told, is unusual among competitive sports in that there has been no fundamental change in the rules for almost a century and that, as with cricket but unlike most other team sports, statistics on the performance of individual players have real meaning. So it is possible to compare the players of today with those of previous generations in a meaningful way.
According to my US sources, the term is first attested in the early 1980s, and derives from the acronym SABR, short for the Society for American Baseball Research, whose members collect and analyse baseball statistics. Several compounds are known, such as sabermetrics for the discipline. This is a recent appearance:
You know, Davey’s always been on the cutting edge. This is a guy who was employing sabermetrics before there was even a sabermetrician. Like I said, he’s a thinking man’s manager, he’s a player’s manager, he’s a guy who loves the game, knows the game inside out, and he has a track record that’s second to none.
The Washington Post, 26 Jun. 2011.