intelligence quotient is an abstract measure of intelligence derived from standardised testing. when used correctly, it can identify those with an intellectual disability so that they can be provided with equitable support. when used incorrectly, it can be (and has been) used to deny healthcare, education, and employment. IQ has also been used to justify scientific racism. there is also arguments over whether IQ as a measurement is accurate enough to be beneficial at all.
if you're saying someone has a low IQ without evidence to support your position (i.e. a standardised IQ test), you're being ableist.