FLV or Flash Video is a multimedia container format maintained by Adobe Systems. It was once the staple of how most media was delivered on the internet in the early 2000s.
Although replaced in the modern streaming world, FLV files do exist and converting to MP4 is easy when encoding with libx264:
$ ffmpeg -i input.flv -vcodec libx264 output.mp4
Tip: Instagram and Twitter do not support FLV video uploads.