On axis just means the speaker is pointed at your head or as close as possible. Most tweeters/drivers perform best directly on axis and start to wane in wide-range FR the further off axis they go. Here's an example using the FR curve of the Vifa XT-25 ring radiator tweeter(courtesy of Madisound):
Axis response is color-coded, so the black line is directly on-axis, purple is 15 degrees off axis, and red is 30 or 45 degrees off axis. Obviously the more off axis you get, the more attenuated the upper range will be even though they all play consistenly until about 2K.