Someone who's credible is honest and believable. A pathological liar, for example, might not be the most credible witness for your case against the bank robber.
Similar to words like reliable and plausible, credible is an adjective that comes to us from the Latin credibilis, meaning “worthy to be believed.” A credible reputation is often earned through consistent good behavior and an overall trustworthy personality. Spill a secret you promised to keep, or lie under oath in a courtroom, and you lose all credibility.