calibration is basically ensuring that all of the speakers sound the same volume, at the main listening position. You can do it by ear, but it's best to do it with a sound-level meter. You just hit the calibrate button and it sends white noise too all of the channels, rotating from FL, C, FR, SR, SL, FL, and so on. I guess for a 6.1 system it'd do the rear one too. Anyway, these are all put out at the same volume by the receiver, but due to differences in the speakers and their distances from the listening position, they can seem to have different volumes. So you want to even these out. You can adjust all of the speakers in one-dB increments until they're all approximately equal. Or, if you're desperate to get real flash-bang surround sound to impress everybody, you can bump the rear speakers up 5dB or so 😀 Again, it's best to do calibration with a SPL meter, but if you don't have/want one, you can do it by ear. If I were you, I'd spend the money on rear speakers before I bought a SPL meter 😉
The stuff that Prong lists is the next step. AFAIK, the tones on these discs are generally better than the ones generated by your receiver (unfortunately I don't have any of those discs). They also let you calibrate your TV's contrast/color/etc. so that your TV displays as close to 'spec' as possible.