When a story stops dead to explain the world's history, rules, or lore in a big block of text instead of weaving it into the action.
Infodumping worldbuilding is the habit of pausing your story to deliver a lecture about your world. It's the three-paragraph history of the kingdom before anything happens, the character who explains the magic system to someone who already knows it, or the narrator who breaks away from the scene to teach the reader about elvish customs. Some amount of exposition is unavoidable, but an infodump is exposition that has lost all sense of timing and proportion.
This is probably the single most common worldbuilding mistake, and it's the one that kills the most manuscripts in their opening chapters. You've built a fascinating world and you want the reader to appreciate it, so you explain everything upfront. But readers don't come to your book for a textbook; they come for a story. Learning to deliver worldbuilding without infodumping is the difference between a reader who turns pages and one who puts the book down.
Even Tolkien infodumps occasionally (the Council of Elrond is basically a lore lecture), but his world is rich enough that many readers forgive or even enjoy it.
Herbert largely avoids infodumping in the text by using epigraphs, appendices, and glossaries to offload world detail, keeping the narrative lean.
Uses the frame story of Kvothe telling his own tale to justify exposition, turning potential infodumps into character-voice storytelling.
Start with a character doing something interesting. Reveal the world through their actions and reactions. Readers will follow along.
Readers are smart and patient. A little confusion is fine; it creates curiosity. Give them just enough to follow the current scene.
Distribution helps, but the real fix is cutting. Most of your worldbuilding should stay in your notes, not on the page. Only reveal what the current scene demands.
Take a piece of worldbuilding you love (a magic rule, a political system, a cultural tradition). Write it as a blatant infodump: three to four sentences of pure explanation. Then rewrite the same information as a scene where a character encounters or uses that world element. Compare the two. Which one makes you want to keep reading?