A waterfall release means you release several singles leading up to your album, with each new single including the previously released songs. This way, older tracks get renewed attention and generate more streams.
Example:
Release = one song
Release = the new song + the first song
Release = the new song + the two previous songs
How to set up a waterfall release:
Add the new song first in your tracklist.
Then include the songs that were previously released. (EPs can also be released this way.)
Make sure all metadata matches the original releases exactly, including:
Song title
Main and featured artists
Composers and lyricists
ISRC
Identical audio file