Question: I want to ask what is the little numbers on top/over any random number? For example, 75 in² and 125 ft3. What is it called and what does it mean is my question. I couldn't find anything online since I don't know what it is called.

Answer: 
In terms of formatting, numbers and letters that appear in the top half of a line are in *superscript*. Similarly, numbers in the bottom half of a line would be called *subscript*. "Super" and "sub" describe their position over or under the main text. 

In your examples, these superscript numbers stand for a mathematical exponent or power as applied to a unit of measurement. If I were reading it out loud, I might say "seventy-five inches squared," or (more clearly) "seventy-five square inches." It means that I am talking about a unit of distance extending in two dimensions. It's a measurement of surface area, just like houses can be measured in square feet (ft2) and land can be measured in square miles (mi2). The cubic version (in3) would refer to a measurement across three dimensions, or a measurement of volume. 

Other measurements can also be squared or cubed, especially when doing calculations in STEM fields, but square inches/feet/miles are the most common examples of this in the US.
The general term for a number (or other text) written like this is **[superscript](https://en.wikipedia.org/wiki/Subscript_and_superscript)**.

The term in2 refers to [square inches](https://en.wikipedia.org/wiki/Square_inch), which is the amount of area in a square with sides an inch in length.

Likewise, if it were in3, it would be referring to [cubic inches](https://en.wikipedia.org/wiki/Cubic_inch), which is the amount of volume in a cube with side lengths of an inch.