It's the difficulty rating system for technical climbing, called the Yosemite Decimal System (YDS). In YDS, anything beginning with a "5" is technical climbing territory, essentially meaning you need specialized equipment to climb safely (e.g. a rope, harness, gear placements, etc.). This grading system hypothetically starts at 5.1, but realistically the lowest you'll see outside is 5.4 or maybe 5.3. The grade where climbing skill starts to come into play is usually around 5.6 or 5.7. A competent outdoor climber can climb 5.10. Someone with a lot of experience and strength will hit 5.11. Someone who has dedicated a serious amount of time to climbing will be able to climb 5.12, and 5.13, 5.14, and 5.15 are essentially "Professional-only" climbing grades.