Note that these descriptions apply to an individual without exceptional technological or circumstantial assistance. Bonuses from various sources will modify your base level of skill.
Skill 0: You are literally incapable of doing this. A human’s Fly skill or a horse’s Use Electronics skill. A character could only have one skill this low at character generation, and should think long and hard about the implications of it. Characters who being play with Skill 0 in something are not just currently but forever incapable of performing this task due to ingrained limitations (without help, no matter how hard you try you cannot practice or learn to Fly), and this skill can never be raised unless the character is cured of whatever is holding him or her back.
Skill 10: You are significantly below average at this but not entirely incapable of doing it. An Amish person arriving in a big city might have this level of competence in Knowledge (Streetwise) and Networking.
Skill 20: You are below average at this but not to a degree that would mark you as handicapped. An office worker’s Athletics or the Use Electronics skill of a middle-aged parent.
Skill 30: A baseline of competence that one might expect from the average person not using this skill professionally or seriously as an amateur. The basic level of skill required to easily complete daily tasks.
Skill 40: You are slightly better than average at this skill, due to small amounts of practice, a slight natural aptitude, or luck. The Athletics of a healthy man who played high school sports regularly ten years ago.
Skill 50: You are noticeably better than average at this but not so much so that people would assume it’s your job or your calling. The Use Electronics skill of a gamer nerd (but not a computer programmer), an amateur Athlete, the Knowledge skill of a “buff” who is not in academia.
Skill 60: You have the minimum of what would be considered professional skill. The Craft skill of a mass-market artist, the Athletics of a solid college athlete or a third-string NFL quarterback, the Deception of a daytime TV or infomercial actor.
Skill 70: You are professionally skilled beyond the minimum standard of competence, and your prowess is worthy of respect.
Skill 80: You are outstanding and are probably in the top 100 living people in your field.
Skill 90: You are legendary in this field and are one of the top 10 practitioners alive, and perhaps one of the 100 best of all time.
Skill 100: The Ubermensch. The end of evolution. Compleat. Only one person ever has had this level of skill. If someone else ever reaches 100, the previous perfect person was, immediately and retroactively, only ever a 99.