In some cases, the file type extension might differ from the actual file type of your file. This means it could have an extension like .mp3, but actually be of the type AAC. The so-called FFmpeg program can detect the real type of a file independent of the extension. To do so, it uses the so-called ffprobe which reads the first few seconds of a file to find any information about the actual file type. Once this is done, FFmpeg assigns a score to its finding. This is the Probe Score.
If the Probe Score is low, it is likely that the file has another type than the extension would make you believe. A high Probe Score, on the other hand, suggests that the file has the same type as the extension. The maximum Probe score is 100 which means that FFmpeg is, basically, 100% sure that the file type extension shows the real file type. When the score is low, one could extend the ffprobe duration to find out more about the file and see if more reading would generate a higher score. If not, the file type is likely to certainly different from the extension.