For minor skill checks, you need a 15% or higher skill rating for an automatic success — no dice needed. These are situations where you have plenty of time and no risk, like taking a photograph of your house or reading up on Greek history. If you do not have a suitable skill, you may attempt a minor skill check by rolling against the appropriate stat instead, to just barely squeak by the task.
Higher skill levels are important for significant skill checks. These happen when you aren’t in an intense situation such as combat, but there’s still some pressure on you or what you’re doing has a high element of uncertainty. Spending a day hacking an unfamiliar operating system, keeping an eye on your husband in a big crowd, or studying for a weekly test could qualify. For these tasks, you have a strong success if you roll under your skill level. If you fail the roll, you get a weak success if that roll is still under the related stat. You only fail completely if you roll above your stat. If you do not have a suitable skill, you may attempt a significant skill check by rolling against the appropriate stat instead for a weak success; for this roll, your stat is reduced by 30.
Major skill checks are the big ones, and in the occult underground they happen all the time. All combat rolls are major checks, as are any actions you attempt under stress and risk. If it really matters, it’s a major check. You only succeed if you roll equal to or under your skill level. If you do not have a suitable skill, you may attempt a major skill check by rolling against the appropriate stat and hoping for a Hail Mary.