pH is just a way to say how acidic or alkaline something is. It works the same way the richter scale does for earthquakes where 7.1 is ten times more than 7.0.
KH is a way of measuring the buffering capacity of the water. What does that mean? If the pH of the water is 7.0 and I add a teaspoon of something acidic (like vinegar) then how much buffer I have will determine how much the pH drops (becomes more acidic). If I have low KH, then the vinegar will make the water's pH drop a LOT. If I have high KH, then the pH might not change at all. Lots of things affect the KH - adding CO2 gradually makes the KH level go down, the biological filter activity will do the same. You can easily increase the KH if it gets too low with a tiny amount of baking soda. Note: because there is always a mix of acidic and alkaline things in your tank, increasing your KH will raise your pH (i.e. the water is more buffered against change in the presence of acids).
GH (how hard your water is) is the KH plus things like magnesium and calcium. Not super critical unless it's really low and then you'd want to dose magnesium by adding a tiny amount of gypsum salt and add a little calcium. KH test kits typically test for both KH and GH.
RO/DI stands for Reverse Osmosis/De-ionization. It's a membrane filter that is so fine that it filters things like calcium salts out of the water. You know all that white crud that builds up on your shower head and coffee pot? That's it. That's also the stuff that builds up your KH. So if you use RO or RO/DI water, then you have to be very careful with your KH and pH levels. Most people get stuff to add enough KH back in. Or you can just mix a little tap water into the RO to get the level you want. A lot of people don't bother with RO water unless their tap/well water is wonky or they have extremely picky fish. Not for beginners.
How to use the CO2 charts: Test for pH. Test for KH. Then use both values to see how much CO2 you have. Unless the KH is really low (below 2-3 °dH), don't adjust it until you have a LOT more experience! DO NOT MESS with adjusting pH! If the KH is really low, then add maybe 1/4 teaspoon a day of baking soda until it is 4 or 5 °dH. Gradually increase the CO2 until you get 20-30ppm. This will lower the pH and KH so make sure you test regularly so the levels of the pH don't drop too low. How much CO2 to add will depend on tank size, how you are diffusing it into the tank, circulation, etc. Make sure you have at least a little surface movement.
P.S. most KH test kits give you the results in mg/L (which is the same as ppm). Every CO2 chart uses °dKH. Here is the simple way to convert from mg/L to °dKH:
mg/L * 0.056 = °dKH
40 mg/L * 0.056 = 2.24 °dKH
80 mg/L * 0.056 = 4.48 °dKH