The Planetary Nebula Luminosity Function (PNLF) is a model consisting of a (non-normalized) Probability Density Function of the Luminosity of Planetary Nebulae. With it, planetary nebulae are used as Standard Candles, using the fact that there is a (rough) maximum brightness of planetary nebulae, which can be used in the manner of distance-determination of the Brightest Star of a galaxy, the Globular Cluster Luminosity Function, or a Brightest Cluster Galaxy in a Galaxy Cluster.
Typically, observation is carried out for an [O III] Forbidden Line (Wavelength 500.7nm), which is produced by planetary Nebulae and can be observed from the ground, and established as from a planetary nebula, i.e., other possible sources eliminated. By observing a number of them in a Galaxy, the distance to the galaxy can be estimated based upon the brightest of the group. The function is typically stated in a way that relates a Distance Modulus to the Apparent Magnitude of this line from the galaxy's brightest planetary nebulae.
Planetary nebulae occur throughout galaxies, including easy-to-view portions, which makes them better for this than some other such standard candle candidates within galaxies.