Absolute Magnitude (M) is a measure of a star's intrinsic brightness, i.e., how bright it would appear at a standard distance of 10 parsecs.
Apparent Magnitude (m) is the brightness with which we observe the star from Earth, which depends on the distance to the star: the farther away it is, the dimmer it appears.
\( m = M + 5 \log_{10}(d) - 5 \)