To dim is to become less bright. Astronomers have found that a start shines slightly less brightly when another object passes in front of it and they can use the measurement of this change in light, the 'subtle dimming', to detect other star and planets.

To add to Neal's answer, "subtle" means faint, slight, just barely detectable. "Can you taste the lemon in this recipe? I only put a few drops in, because I wanted it to be subtle." The "b" isn't pronounced, by the way--it's pronounced as if it were spelled "suttle."