SPL: sound pressure level--ie the amount of decibels.
Deciel (or dB): a measure (logarythmic) of sound. Loosely speaking volume.
We all talk about SPL, because ideally when fed the same signal, all of your speakers should be putting out the same dB level. The meter lets you measure this. To provide the signal, either your receiver uses pink noise (a balance spectrum of sound, which sounds a whole lot like static) or you have a DVD that provides the pink noise for each channel.
Your receiver should have a setting you adjust for each channel (speaker) to control the individual volume of that speaker.
I hope that clears a few things up.
