Sabermetrics are confusing. The above clip from the Moneyball actually does a pretty good job of explaining what the A's were doing back in 2002. Sabermetrics have evolved a lot since then but a lot of baseball fans still don't understand some of the basic changes to how we view stats.
A player's batting average used to be their most important statistic. Back in the early 1900's when Ty Cobb and Honus Wagner played the best players were considered the ones with the highest average. In theory it makes sense. This was before players really hit for any power and pitchers weren't walking guys so your ability to get a hit defined your worth.
However in the next 50-75 years two things happened. The first is players started hitting for more power. A home run is obviously more valuable than a single but batting average doesn't account for that. The second thing was pitchers started walking people. The Moneyball A's are given a lot of credit for realizing the value of a walk but people such as Bill James and Earl Weaver understood this years earlier. Basically the point of baseball is to score runs. In order to score runs you need to get on base. One way to do is that to draw a walk. For the longest time it was considered a mistake by the pitcher if he walked someone. While that can still be the case we now also view it as a skill by the batter. It's why Joey Votto has such a high OBP. He draws tons of walks.
Sabermetrics users also understand another flaw in batting average: luck. How often do a player hit a blooper that falls for a single? And how often does a player hit a screaming line drive right at the third baseman? Over the course of a season these both can happen a lot. People used to just say "it will even out". Turns out it doesn't even out, which is why batting average statistics can fluctuate so much on just a year to year basis. Often times it is because the player is having a good or bad season but there is clearly also luck involved.
Anyways the A's realized that batting average was overrated and OBP was underrated. Basically it became better to hit .260 with a .350 OBP than hit .290 with a .310 OBP. Batting average is still important don't get me wrong. Look at the leaders from 2015:
Most of these guys are good players, so clearly batting average still has value. It's just important to remember if the player doesn't also have a high OBP and/or hit for power, then average doesn't have as much value as it once did.