Slope can be written three ways that all mean the same thing - as a ratio like 1:12, as a percentage, or as an angle in degrees. A wheelchair ramp spec, a road grade sign, and a level's degree reading can all describe the exact same slope - genuinely useful to convert between them correctly, since a ramp built to the wrong number is a real accessibility and safety problem.
A ratio like "1:12" describes rise over run directly - genuinely useful for construction, since it tells you exactly how much horizontal ramp you need per inch of height. Percentage grade is the same math expressed differently (rise/run x 100) and is what road signs and most spec sheets use. Degrees measure the actual angle from horizontal, which is what a digital level or angle finder reports. All three are legitimate ways to describe an incline - the trap is assuming a "12" in one system means the same thing as a "12" in another, when a 1:12 ramp and a 12-degree ramp are dramatically different steepness (12 degrees is roughly a 1:4.7 ratio, more than twice as steep as ADA allows).
Take the arctangent of the percentage divided by 100. A 10% slope is arctan(0.10) = 5.71 degrees. A 100% slope is 45 degrees, not 90.
It means one unit of rise for every twelve units of run, which is about 8.33% or 4.76 degrees. It is a common accessibility standard for wheelchair ramps.
Each of these has its own page with the exact factor, a reference table and the working shown. The converter above handles any of them too - these exist so the specific answer is one click rather than a search.