It’s kind of un-intuitive but actually simple. (edit: it took me ages to understand it to start with!)
In this view
I’m on the “R” tab. That means I’m controlling the amount of red in the pixels.
The three sliders let you control the amount of red in the red, green and blue parts of the image.
I find it useful to bear in mind that in general, it’s best to keep adjustments balanced.
By that I mean that if, in the screenshot above, I slide the “B” slider down to say -0.200 to reduce red in blue areas of the image, I will slide the “R” slider up to +1.200. Otherwise I get an overall color cast.
But this way, I get a bluer sky (actually more cyan as that’s the opposite of red) and the reddish parts of the image just get a little more red.
To take another example, if I wanted the red areas to be yellowish, I would move to the “G” tab, and increase the “R” slider to increase the amount of green in the red parts of the image, making them more yellow (R+G=yellow), but I would compensate that increase by reducing one (or both) the the other sliders in the “G” tab.
I know this is sort of off topic, but maybe it helps a little? 