When most cameras save a RAW file they also generate a preview JPG and embed that into RAW file's data. It's that JPG you're seeing when you preview photos on the back screen. It's also used as the preview in some imaging apps to speed things up--processing the RAW information and then interpreting it takes time and processor power, after all.
There are times when being able to extract those embedded JPGs can be useful. You might want to get the version with the camera's built-in effects applied (which don't apply on the underlying RAW image--just the JPG preview). You might want to speed things up for culling or processing, because it's quicker to read a JPG that already exists than generating a new one from the RAW data. You might want to quickly share some lighter-weight JPGs without messing with RAW processing software. Or if the underlying RAW image data has become corrupted, it might be a viable way to salvage at least the preview image.
Here are two ways to extract those preview JPGs from the RAW files.
The ERawP app (or Extra Raw Preview) is developed by Fulvio Senore. It's free and cross-platform, with versions for Windows, Linux, and Mac.
It's very simple, and there are no bells and whistles. You simply choose the directly of the original images and set the folder where you'd like to save the extracted JPGs.
The documentation is sparse and it's not clear how frequently it's updated to work with new revisions to the propriety RAW formats—it seems to be using the same version, 1.0, for the past couple of years. For the common formats I've tried it works well, but your mileage might vary if you're using a less-common or newer format.
You can download it here.
Instant JPEG from RAW
Instant JPEG from RAW, from Michael Tapes Design, produces basically the same end result but gets there a slightly different way. It installs new items in the context-sensitive menu that are accessible when you right click on the files in Finder or Explorer. You can work with individual files or choose multiple files at once.
It's free, and there are versions for Windows and Mac.
It's still pretty bare bones, but you do have a couple more options. You can extract the images at their native size or resize them smaller. You can also choose whether the extract JPGs are saved in the same folder alongside the RAW originals or in their own folder, with or without a suffix identifying them as being extracted by the service.
This feature is also built into PhotoMechanic. You can find it under Tools > Extra JPG Previews from RAW Photos. But given the price of PhotoMechanic, this option is probably only practical if you're already using PhotoMechanic (or plan to use it for other things as well--it's very powerful).
One app that is especially good at working with the embedded previews is PhotoMechanic. It's one of the reasons the app is lightning fast.
In its most recent version, Lightroom Classic CC has added this feature, at least in part. When you first import images, it initially reads the embedded previews before it's had a chance to replace them with new preview image.