Excuse me, I meant are there a lot of commands I need to know with the terminal or a text editor? I want to learn how to change the source code but it sounds complicated.
On the contrary, changing the source code is easy.
I want to know what the text in the source code means & what it does.
That's more like it. Knowing what to change, and what to change it to, is the difficult bit.
It sounds as if you are taking about a whole Linux distribution rather than just the kernel. This consists of millions of lines of source code that people have been working on for twenty years or more. If you don't know what any of it means, your chances of successfully modifying it at present compare unfavourably to those monkeys typing Shakespeare.
1. You need to learn how to program in C.
2. You have to learn how to program in x86 assembler.
3. You need to learn the coding conventions used in Linux source code and the functions provided to programs by its various libraries.
4. You need to study the source code of the Linux kernel and the user programs to understand what it does. You'll probably need to read a few books to help you with this.
5. You need to decide which part of the Linux sources you wish to change and what effect you want the change to have.
6. You can then make the change, compile the resulting source, and test it.
7. Repeat 4 & 5 until the changes you make have the desired effect and don't break anything.
8. Repeat 4, 5, & 6 until you have achieved your objective.
Start with point 1. Allow yourself 6 months to a year to become proficient in C. Then you can go on to step 2, and so on.