Sorry for so many C&Ps from Fangraphs but they've had a lot of interesting articles over there lately...err, well...always.
Check it out. Here's an excerpt:
We often talk about "replacement level" players, especially when talking about salary and whatnot. Here's a face we can put on that player now...at least, a pitcher.If you've hung around here very long, you've heard the term replacement level. We often refer to a players performance in terms of his Wins Above Replacement, which is based on the replacement level baseline. Despite significant inroads in acceptance, replacement level can still be one of those murky things to try to explain to someone. Fans, and even some GMs, will often find it hard to believe that you can get decent performances from guys for the league minimum, and constantly want examples of guys who prove the replacement level baseline.
There's nothing that stands out about Ayala. He's just a run of the mill strike throwing reliever with nondescript stuff. The Wins Above Replacement metric things he's essentially the baseline against which all other relievers should be measured, and MLB teams agree. Ayala's performance is good enough to keep landing him jobs and bad enough for those teams to decide they could do better.
Luis Ayala defines replacement level.