The X-axis of the H-R Diagram indicates the star's surface temperature in degrees Kelvin. The Y-axis, on the other hand, indicates luminosity, or brightness.
Main sequence refers to a roughly diagonal, slightly S-curved line stretching between the upper-left and lower-right corners on which main sequence stars chart. They maintain a predictable relationship between luminosity and temperature: the brighter, the hotter. The upper-right quadrant of the H-R diagram is home to newly discovered red giants while the lower-left quadrant of the H-R Diagram belongs almost exclusively to white dwarfs.