It's probably best explained by example:
Imagine there is a question with 2 answers, where answer 1) is thorough and exhaustive, while answer 2) includes a good point, maybe even an example image or a joke, but in general does not cover the whole question. Answer 2) is currently the popular one and has the most up votes.
In absolute terms I find that 1) is the best answer and 2) is an interesting addition to it and I guess I should upvote them both, because both are relevant and useful. In relative terms I find that 1) is better answer and should be accepted, so should I leave 2) without vote or even downvote it (doesn't really matter in terms of reputation)?
Real world example of good vs popular answer: https://stackoverflow.com/questions/284797/hello-world-in-less-than-20-bytes/284898#284898 (somebody with enough rep on SO could maybe add the total up and down votes for that answer here).