An “IQ” is a score that comes from an intelligence test. The term used to stand for “intelligence quotient,” but it doesn’t stand for anything any more. That’s because the formula for calculating IQ used to be based on division ([mental age / actual age] x 100), which produces an answer called a “quotient.” The mathematical formula for calculating IQ has changed, but the name has stuck around.
The average IQ is 100, and people who score higher than this number are smarter than average, and people who score below 100 are less intelligent than average. The further a person gets from 100, the further from average they are.
There is over 100 years of research showing that IQ tests do indeed measure intelligence. It’s impossible for anyone to read it all in a lifetime, but up-to-date summaries are available here and here. These scores predict educational outcomes, work and occupational outcomes, health and longevity, and many other outcomes. IQ tests are some of the best designed and highest-quality tests in all of psychology.