In the world of BitTorrent, "pieces" are the small, equal-sized chunks into which a large file is divided to allow for efficient sharing and verification between users.
This often happens if the final piece of the file is missing from all available seeders.
Most clients like qBittorrent or Transmission allow you to "Download first and last pieces first," which helps some players preview a video file before it is fully finished.
If a piece is corrupted during download, your client will detect it via a hash check and try to download that specific piece again.