The slope of a roof, expressed as rise over run — e.g. 6:12 means 6 inches of rise per 12 inches of horizontal run.
Pitch matters for both materials and labor pricing. A steeper roof has more surface area than its footprint suggests (a 6:12 is roughly 1.12× area; a 12:12 is 1.41×) and is slower and more dangerous to work on — so labor rates step up at steeper pitches.
Low-slope roofs (under 2:12) typically can't take asphalt shingles and need a membrane system instead.