A tiny swap, a very different snack
A spoonerism swaps sounds between words, usually their opening sounds. It can turn an ordinary phrase into something delightfully silly.
Take jelly beans. Exchange the first sounds and you get belly jeans. Your snack has apparently developed a wardrobe.
Follow the sounds
Keep the rest of each word in place. Move the j sound from jelly to beans, and the b sound back the other way: jelly becomes belly; beans becomes jeans.
Cambridge Dictionary describes the accidental version: the first sounds get exchanged while someone is speaking, producing an unintended, often funny meaning.
From weather to the road
Here is another pair: light rain → right lane. Swap the opening l and r sounds. A weather report suddenly sounds like driving directions.
Notice that rain becomes lane, with a different spelling. We are following pronunciation; the spelling can change too. You can also make these swaps on purpose as a word game.
A name, and a different kind of puzzle
The term comes from William Archibald Spooner, a British clergyman and educator, as Merriam-Webster explains.
Unlike anagrams, which rearrange letters, this joke works through sound. Say both versions aloud and listen for the moment a sensible phrase takes a wrong turn.