These zip files are typically PGN (Portable Game Notation) collections structured so you can practice tactics specifically related to certain openings:
: Puzzles are grouped into 5 main folders (A through E) and subdivided into 495 specific PGN files by ECO code. Lichess Puzzles By Eco zip
The phrase refers to a popular community-curated dataset where millions of Lichess puzzles are organized into zip files based on their Encyclopedia of Chess Openings (ECO) codes . What the Zip Files Contain These zip files are typically PGN (Portable Game