According to the fine-tuning argument, the fact that there's this universe in which various constants had to have very precise values in order to make life possible requires an explanation which can be found either in a cosmic engineer (which would pretty much have to be a god) or a multiverse (which expands our explanatory resources). Some people say that fine-tuning can be answered by appeal to the anthropic principle. The anthropic principle is a particular manifestation of the observer selection effect. In this case, we would have to find ourselves in a life-permitting universe since a universe would have to be life permitting in order to contain us. We couldn't very well observe ourselves in a universe incapable of supporting life. So there is an observer selection effect that makes it possible to only observe life-permitting universes.
Some people think this answers the fine-tuning problem. Since we could only observe a universe that is life-permitting, it shouldn't be any surprise that that's exactly what we observe. Therefore, there's nothing that requires explanation.
The problem, though, is that the anthropic principle only works if you also invoke a multiverse. If there were a multiverse, and we were asking why we observe a life-permitting universe instead of a life-prohibiting universe, the anthropic principle would answer that question. Even if life-permitting universes are rare in the multiverse, they are nevertheless the only kinds of universes that can be observed since they're the only kind that can support observers.
But the question raised by the fine-tuning argument isn't why we find ourselves in a life-permitting universe instead of a life-prohibiting universe. Rather, the question is why we find ourselves at all given how unlikely it is that a universe that could support us would exist.
Consider the firing squad analogy. I got this from somebody else, but I don't remember who. Sorry about that. Anyway, imagine you're in front of a firing squad, and after they all fired, you're still alive. That requires some explanation. Why are you still alive? It wouldn't do to say, "Well, there's nothing here that needs to be explained since I would have to be alive in order to be asking the question." That doesn't answer the question. The question isn't why you're in front of a missing firing squad instead of a hitting firing squad. The question is why you're still alive at all since the probability was against you.
The puddle analogy is kind of like the anthropic principle. The puddle finds itself in a hole that seems perfectly suited to it. We know, however, that it's actually the puddle that has conformed to the hole rather than the hole that just happens to be suited to the puddle. In the same way, a lot of people say the universe wasn't made for us; rather, we conform to the way the universe already was. We are the way we are because the universe was the way it was, so there shouldn't be any surprise that we find ourselves in a universe that's perfectly suited to our existence.
It's certainly true that we have come to conform to the way the universe actually is. But remember, the fine-tuning argument isn't about why life turned out one way rather than another way. Rather, it's about why there's life at all, or why the universe is life-permitting at all. In the puddle analogy, the question shouldn't be why the puddle and the hole are perfectly suited for each other, but why a hole should exist at all that could contain a puddle.
The puddle analogy is similar to another objection to fine-tuning which is just based on a misconception about the fine-tuning argument. Some people take the fine-tuning argument to be about life as we know it. The assumption behind this misconception appears to be that if we just tweak the constants a little, we'd get a different kind of life. But that isn't the claim. The claim, rather, is that without fine-tuning, no life whatsoever, be it ever so exotic, would be possible since life of any kind (or at least any physical kind) requires complex chemistry, and complex chemistry requires fine-tuning.
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