It's just a ratio between a triangle's lengths.
Yeah- it's the inverse of the original trigonometric function.

Take a read of this stie: https://www.mathsisfun.com/algebra/trigonometry.html
It explains it pretty well.

Also, asin, acos, and atan are the opposites of sin, cos, and tan, although more specifically, they are the inverse.

Like this:
sin(30°) = 0.5
asin(0.5) = 30°

We refer to each of those functions as triginometric functions - each of them are related to right triangles. Sine, cosine, and tangent each represent different ratios of the side lengths of the said triangles with respect to a given angle of a triangle.

Arcsine, arccosine, and arctangent are inverses of their respective functions. Sometimes, they're referred to as “inverse sine” and so on. They evaluate the actual angle of the triangle with respect to the ratio of two side lengths of a triangle.

The Scratch Wiki has a quick explanation of what this is here: https://en.scratch-wiki.info/wiki/Trigonometry
If you want to have a very in-depth explanation of basic right-angle trig, you can read more about it on Lumen as well: https://courses.lumenlearning.com/boundless-algebra/chapter/trigonometry-and-right-triangles/

Thanks for all of the feedback!