Definition: The term you're asking about is "Matthew 3:17." It's a verse from the New Testament of the Bible that appears on page 206 of Mark's Gospel (Matthew) and includes verses 15-18:

"Then Jesus said to them, 'Listen to me; I am sending My one and only Son, the Word, into the world. He will not cease until it is finished.'" This verse has become famous for its phrase "Matthew 3:17," because it's often used in expressions like "it's Matthew 3:17" or "the word that Matthew 3:17 said." An example sentence using this expression might be, "I'm feeling a bit of a Matthew 3:17 right now. I've been trying to write a novel for a long time, and every night before I go to bed, I sit down to work on it with the help of Matthew 3:17." This example sentence goes against what we might expect from this verse, which is about Jesus coming into the world to serve humanity. However, when used in a humorous way, it can be taken as a play on words or a playful reminder that sometimes the simplest things can hold the most profound meaning.