It's like a giant electric toothbrush, but y'all, I own one and it REALLY works. You literally see the dirt come up and the grout lighten with just 30 seconds of use. They make a specialty grout head you can buy for it, but TBH the default brush works just as well, unless you have incredibly narrow grout lines.
Promising review: "Last night I was reborn. Everything I'd experienced up until that point suddenly was just 'before.' The present and future seemed like a beautiful, promising 'after.' 'What is the cause of this renewed zest for life?' you might ask. It was the use of the Rubbermaid Reveal Power Scrubber on my shower grout. It elevated a simple baking soda and water solution to alchemical proportions. This thing is nirvana. My pink-tiled shower, which never looks clean no matter how many hours of backbreaking and bicep-burning labor I put in, now looks bright, clean, full of promise. An otherwise disgusting and dated purely utilitarian part of my home is now art. This this is absolutely incredible. Buy it, see for yourself. 11/10 would recommend." —Amazon Customer
Get it on Amazon for $15.99.
If you want even more info, check out my full Rubbermaid Reveal Power Scrubber Review.