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.
**Exponent** 

> 
> 3.6 Derived Units— Derived units are formed by combining base units according to the algebraic relations linking
>  the corresponding quantities. Symbols for derived units are obtained by means of mathematical signs for
>  multiplication, division, and the use of **exponents**. For example, the SI unit for speed is the meter per second
>  (m/s or m·s–1 and that for density is kilogram per cubic meter kg/m3
>  or kg·m–3). Most derived units have only
>  their composite names, such as meter per second for speed or velocity. Others have special names, such as
>  newton (N), joule (J), watt (W), and pascal (Pa), given to SI units of force, energy, power, and pressure (or
>  stress), respectively. 
> 
> 
> 

SAE Technical Standards Board: Rules for SAE Use of SI (Metric) Units, Rev May 1999.
<https://www.sae.org/standardsdev/tsb/tsb003.pdf>