THM files are small files often created by the camera to act as thumbnails for images, and more often videos. They have a .thm extension following the file name.
THM files make it easier to browse a series of images and videos in playback without having to load the actual media file in memory.
Video files have the exact same name as .thm files, but with a video extension such as avi.
The THM file is the first frame of the video. It is the initial still image you see when previewing a movie in a camera.
THM file is a small JPEG
For videos, THM files often are used to store data much in the same way as is done with EXIF data for still images. This is because the THM file is actually a small JPEG with a different extension.
If you change the .thm extension to .jpg, the file will change to a JPEG image (for example, change. MVI_8233.thm > MVI_8233.jpg).
Most cameras will not playback videos if the THM file is missing or damaged.