It's a psychological concept used to approximate someone's psychometric intelligence, which has been shown to be useful in predicting job Performance and various measures of academic success. The popular, but ancient, method of calculating IQ, or intelligence quotient, was to divide someone's mental age by their actual chronological age. Contemporary methods involve plotting one's relative score on a distribution score, derived from a norming group which is usually stratified by chronological age, which allows individuals of all ages to have their IQ's evaluated.
It's tended to be used as a conversational last resort, which is more typical among those with a IQ worth mentioning, or in some cases, rights to brag. Those mentioning an IQ above 140 are usually easily identified as the classical Braggart; Less then .4% of the population are able to earn an IQ this high on commonly accepted IQ tests.
Also, Communication difficulties and other academic or learning problems doesn't mean that someone MUST HAVE A low IQ, Albert Einstein being the common example. I happen to be another.
My IQ is 145, or at least that's what the graduate psychometrist told me. No really. I'm not making this up. Despite my admonition in the definition, I, this definition's writer, have an IQ above 140.
Like anyone cares though. Haha.
I'm pretty sure that's going to earn a couple of thumbsdown.