Most tutorials on this site have been designed to build on top on others.
This makes it easier to follow them in specific sequences.
A series represents this kind of sequence that has been premade with a specific kind of output and topics in mind.
A series is like a playlist.
The strong suit of Houdini is its Procedural workflow. But, since its not the way most software work, lets take a few minutes to quickly understand what Procedural workflow Implies.