You use these charts to learn the brightness levels of various reference greys (the greyscale chart and Histogram Reference Tools) and to learn the RGB values for various colors and how the camera sees them (the color chart). The Histogram Reference Tools show you the grey levels at the division lines on the camera Histogram so you know what the brightness level is at each line.
The Nikon and Canon Histogram Reference Tools show the distance in EV between the histogram markers at the bottom of each chart. The Nikon chart shows the number of RAW data bits at each division and the percentage of data points that each region represents.
The 3rd linked file shows a 5% greyscale in the center, with the Canon Reference greys on top and the Nikon Reference greys on the bottom of the central chart.
The 4th linked file is the large version of the greyscale at the bottom of this page. It is useful for learning the levels of dark greys (and for checking to see that your monitor brightness and contrast is set optimally... you should be able to just barely see the 0-5 division on a superb monitor and the 5-10 division on a very good monitor, while still seeing a strong division between 242 and 255 on the upper greyscale).
The Color Reference Chart shows Grey 128, Cyan, Magenta, Yellow and White strips on top and a grid with several reference greys and incremental color sections. Each incremental color section alters only one color (R, G or B) through the range 0, 77, 153, 204, 255. This shows you how RGB values affect color. The number at the top of each color patch shows the actual RGB value of that color, and the number at the bottom of each patch shows the equivalent grey that the camera sees through the Beyer Pattern Filter over the sensor. You will learn the equivalent grey for various colors and will understand how to expose for those colors. (This will help you understand why full saturation red or blue is often overexposed, as an example). This file is in the AdobeRGB color space so the colors will be accurate when opened in a color-aware program such as Photoshop. They do not display accurately in non-color-aware web browsers.