Simple as I can explain it: Water has a saturation point. There is a finite quantity of whatever that will dissolve in it. We did an experiment once in high school chemistry with a beaker of water and a pile of sugar. Dump in some sugar, stir. Repeat. It gets syruppy and at some point, the water is saturated, any more sugar just sits on the bottom in a pile, undissolved. Then by heating, a lot of that pile dissolves. Because temperature affects the Ksp. That's shorthand for solubility product constant.
Now, there is also a Ksp for Calcium. You can dissolve so much into the water and at some point, it won't absorb any more. Tinkering with pH and temperature may get it to dissolve more. Likewise, tinkering the other way will cause a clear solution to start precipitating calcium. Cloudy at first, and then it will settle to the bottom of the beaker.
Move to the real world - your pool. Depending on temperature, pH, TA, and a few other variables, Calcium will stay in solution (dissolved) or precipitate out - start crystallizing on the walls, in the plumbing, the ladder, whatever. We call it scaling. The CSI predicts which way it will go.
Positive CSI risks scaling. Negative CSI means it will start leaching Calcium out of the plaster walls, etching them and weakening it. Keeping CSI +/- 0.6 means that neither scenario is likely. I can say from personal experience that when CSI gets around -.6, it does dissolve Calcium. Fast enough to see a difference in a week. I've been slowly descaling my pool that way for roughly the past year.
You will see as you play with the numbers on Pool Calculator that the Calcium is not the biggest factor for scaling - pH is. TA can also have a large effect. The difference to CSI between 800 and 850 CH is minimal. However, there will come a time, and I have been there, where CH is so high that the only way to keep it in solution is to have pH out of the comfort zone. Meaning, the pool looks great, but you don't want anyone swimming in it! And that's when it's time to drain some water and refill it.
Does that help any?