A series is a list of levels intended to form a full adventure together. Series are very common on Level Palace, and is usually the way to go if you're a story-driven level designer, or if you want to make a series of classic levels.
The Level Palace site has some features made for series, such as collections: a page for your series where you can upload all your series' levels to.
Additionally, on level pages where the level is inside a collection marked as a series, other levels within that series will also show up in the "See Also" menu.
There have been made hundreds of series on Level Palace, but here are some examples that are on the wiki:
Super Mario Legends (SMF2)
7 Days in Russia (Soviet Edition)