A mentor is a person who trains and guides someone, like the second-grade English teacher who saw the spark of creativity in your writing and encouraged you to become a professional author.
The original Mentor was a character in Homer's long poem Odyssey who looked after Odysseus's family, particularly his son Telemachus, while Odysseus was off fighting in the Trojan War. Today the word is used to describe any person who acts as an advisor or teacher. As a noun, mentor describes the coach who taught you to play baseball. As a verb, mentor is what the coach does to help you improve your swing.