What are streams?
A file (on NTFS) can have more than one stream, where a stream is basically another representation of the file. Most files only have one stream, but NTFS encrypted files, for example, may have more than one stream where one of the streams contains information about the encryption.
The following article has more details:
https://www.2brightsparks.com/resources/articles/ntfs-alternate-data-stream-ads.html