Best is the enemy of better

What does the following sentence mean?

"Best is the enemy of better."

It's a rallying cry for mediocrity, a plea for the ordinary and adequate.

Normally, one sees "good is the enemy of great," which means settling for "good enough" prevents one from attaining the highest levels. In the version you cite, however, the pleas seems to be the opposite: don't consort with Best when you can settle for Better. Hmm. I have a mother-in-law like that....