So imagine a laser beam shooting from the tweeter, straight out. "On-axis" would mean your head would be in-line with the laser beam. Really high frequencies are like a laser beam, and low frequencies are like the light from flourescent tubes (multi-directional).
The freqs from a tweeter can literally shoot right past you or over your head if you are too far off-axis, depending on the design of the speaker of course.
***I suppose the laser beam analogy would be more conical from the tweeter and not as linear, but you get the point!