"About right" is a phrase meaning "approximately correct" or "roughly accurate". English also uses the phrase "just right" to mean exactly correct.

What does it mean to say right?

1 : righteous, upright. 2 : being in accordance with what is just, good, or proper right conduct. 3 : conforming to facts or truth : correct the right answer. 4 : suitable, appropriate the right man for the job. 5 : straight a right line.

Does that sound about right?

The person saying this is guessing that your answer to a question is correct. He/she is not entirely certain, but reasonably assured that your answer is correct.