Amy, spl means sound pressure level, it refers to the loudest sound a
microphone can handle without distortion of one kind or another.
Binaural is a form of stereo recording where the idea is to simulate what the ear actually hears by using microphones spaced about the same distance apart as the human ears and with a baffle of some kind in between. Binaural recordings often do not sound good on speakers, but excellent in earphones. Quasi binaural recordings sound good in both because they tend to cut back on the sounds coming from the rear.
A good binaural recording transmits not only left and right info like speakers, but height above you level as the ear actually hears.