Have you ever mixed up the first letter or sound of two words, like instead of saying "spoonerism" you accidentally say "woonerism"? That is called a spoonerism! It's when you switch around the beginning sounds or letters of two words, often on accident but sometimes on purpose for fun.
For example, if you said "flutter by" instead of "butterfly," that would be a spoonerism. Or if you said "you have hissed all my mystery lectures, and were caught fighting a liar in the quad," when you meant to say "you have missed all my history lectures, and were caught lighting a fire in the quad," that's a really silly and famous spoonerism!
Overall, spoonerisms can be funny and entertaining, but they can also be confusing if someone doesn't realize what you're trying to say. So it's important to use them carefully and appropriately.