Live Caption is one of the most interesting features introduced on the Pixel 4, and it's great that it's coming to the Pixel 3 and 3a line, as well. By default, it only shows you two lines of live-transcribed text, which might be enough in most circumstances. But if you ever need to see more context, you can double tap the overlay to make it display eight lines of text.
Previous video demos already showed this taller UI, but since there was no obvious way to invoke it, we thought that Google changed the interface before the public release. The huge box of text might be a hindrance when you're more focused on the video than the text, but when you need to take in a lot of information, it might just be what you're looking for.