Google's voice commands are impressively powerful, but it doesn't always hear you right. If Google still hears you incorrectly after you've done everything you can to help it understand you better, you can use the "No, I said..." command to fix a word.
The feature is actually pretty good at figuring out what word you're trying to correct. As you can see in the example above, when Google mishears the name "Whitson", I only need to say "No, I said Whitson" to correct it. Google recognises that "Gordon" is OK as is. It's a handy way to correct a small error.