Windows Alternate Data Streams, otherwise known as ADS, were introduced in Windows NT 3.1.1. They were introduced to allow compatibility with the Macintosh Hierarchical File System. Think of ADS as hidden files that are attached to the visible ones. The main reason they are so dangerous is that they are not well known, are generally hidden to the user, and that there are few security programs that can recognize them. Use LADS on your own risk! Scanning directory C:test size ADS in file causes a hard problem when you create an ADS file against a root of a drive.
Source: https://www.bleepingcomputer.com/tutorials/windows-alternate-data-streams/