The simple act of turning balls in play into outs.  Simple, easy, clean concept.

Jonah Keri wrote the following in the NY Sun today in a review of the success the Florida teams are having this year:

 

PECOTA saw the Rays making their biggest improvement on defense. In acquiring slick-fielding Jason Bartlett to replace the mishmash of Brendan Harris, Ben Zobrist, and company, the Rays found a big upgrade at the most important position on the diamond. Evan Longoria has been a big upgrade at third base, moving Akinori Iwamura over to second, where he's become a bigger defensive asset. Subtracting Delmon Young from the outfield has also helped.

Indeed, much of Tampa's perceived pitching problems last year were actually the result of their defense failing to turn balls in play into outs. Last year, the Rays were dead last in the majors in that category (also known as Defensive Efficiency), converting just 66.2% of balls in play into outs. It should have been no surprise, then, to see Tampa post the worst team ERA last year, at 5.53. The Rays' starters were the third-worst group in the game at 5.20; the bullpen was the worst in baseball by a wide margin, at 6.16.

This season, the Rays have jumped to second in Defensive Efficiency, at 72.5%. It's no surprise that their run prevention has also improved dramatically — a 3.68 team ERA, fifth-best in the majors.

NY Sun Article

I really don't think that anyone would dispute that having athletic fielders that can make plays helps out a teams pitchers.  It's great to have a high team fielding percentage, but if the fielders have the range of fire hydrants, it doesn't really do that much good to make few errors if you are letting too many other balls go for hits.

Of course the ideal situation is for a team to have a high DER  and also have a good fielding percentage. I don't advocate looking at only one stat.  I'm just saying that in looking at a team's overall defense, you need to look at BOTH DER and Fldg. % if you want to get a fuller, clearer picture.

Finally, I will leave with this little chart, and leave it for the reader to determine which number seems to correlate more closely to improvement in ERA