If you have a GoPro Hero 3 Black Edition or a Hero 2 or Hero 3 (White & Silver) with the latest firmware update, you may have noticed some .LRV and .THM files appearing on your memory card. Here's an explanation of what these files are just in case anyone else saw these and didn't know what they were.
LRV stands for low resolution video. If you rename the extension to .MP4 instead of .LRV, it will be a lower quality version of the original video you shot.
THM stands for thumbnail. This is a low resolution thumbnail image of the 1st frame of the corresponding video file. For example, if your video file is GOPR0001.MP4, the low resolution thumbnail will be named GOPR0001.THM. Rename the extension to .JPG from .THM in order to open it up as an image and view it on your computer.
The .LRV files are primarily used for video playback on the GoPro App. Since the original GoPro video files have such high resolutions and frame rates, the built-in camera WiFi cannot stream all of this data smoothly. The .LRV files have a much lower resolution and can therefore be streamed smoothly via WiFi to your phone or tablet without too much interruption.
The .THM or thumbnails are displayed when you are browsing through the video files in the App.
.LRV files are not created for all GoPro files though. The are NOT created if you shoot video in 4K or 2.7K or any mode with ProTune turned on.
For 1440k, they are only created when shot in 30 or 24fps.
The .LRV and .THM files are created in all 1080p, 960p and 720p modes except for those shot with a frame rate above (not including) 60fps (so 100fps and 120fps does not generate these files).
Check out the chart below for the list of all video settings and if the App playback is available or not. If App playback is available, this means .LRV and .THM files are created.
The other purpose of .LRV files is to allow quicker and more headache-free video editing on your computer which may not be powerful enough to handle high resolution video files. Refer to this thread for more info on how you can use .LRV files to edit your GoPro footage quicker: Editing your GoPro Videos with .LRV Files