Asexual is the hypothetical lack of sexual impulse, taken up by a number of human beings.
This is an exaggeration. While it is possible for some things to be truly asexual (ie a pebble is asexual) the possibility in humans is unproven.
Usually it simply refers to a person having a negligible / unobserved quantity of sexuality, perhaps due to having below-average sexual impulses.
It's most likely just a different term for being hyposexual except for some reason that term offends them.