Bortle refers to a scale introduced by John E. Bortle in 2001, measuring the level of light pollution in the sky from 1 (a truly dark-sky site) to 9 (the sky as seen from a very big city). Light pollution has a significant impact on what you can observe or photograph, because the stronger it is, the less objects you can see in the sky.

The Bortle Classes
Class 1 → Excellent dark-sky site
Class 2 → Typical truly dark site
Class 3 → Rural sky
Class 4 → Rural/suburban transition
Class 5 → Suburban sky
Class 6 → Bright suburban sky
Class 7 → Suburban/urban transition
Class 8 → City sky
Class 9 → Inner-city sky

Light pollution maps make use of this colour-coded scale to display how light polluted a location is.

Light pollution map