You can't learn all of everything, and you certainly can't do it quickly. What you can do is learn something that you're motivated to learn, but you can't do it by simply reading. You need to work on it.
Math requires work. Euclid said, "In geometry there is no royal road."
The DCT is a complicated thing to "understand", and I'm not sure I do myself. I would guess that the people who wrote the code you're seeking to modify didn't necessarily understand it either - they may simply have coded the formulae they were given.
So don't despair. You can learn things like this, but you can't do it all at once, and you can't do it quickly. You need to start somewhere.
Math requires work. Euclid said, "In geometry there is no royal road."
The DCT is a complicated thing to "understand", and I'm not sure I do myself. I would guess that the people who wrote the code you're seeking to modify didn't necessarily understand it either - they may simply have coded the formulae they were given.
So don't despair. You can learn things like this, but you can't do it all at once, and you can't do it quickly. You need to start somewhere.
What do you really want to know?