Biasing basically means altering the voltage across the valves in your amp. Each amp will have a specific biasing range, e.g. my 50W Vintage Modern likes its valves between 44 to 48 mV. Biasing an amp "hot" means increasing the voltage above the recommended range. I think this means that the sound will break up (distort) quicker but the side effect of this is your valves will also die quicker. Biasing the amp "cold" means decreasing the voltage below the recommended range. As the inverse of hot, this will smooth out your sound and make valves last longer. Perhaps obviously, too hot or too cold will make your amp sound like arse... Although, some [strange] people quite like this! I believe the grading above describes the break up of the tube. I can't remember whether A grade break up first or last but I'm pretty sure that's what the spectrum illustrates.