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.
If you have ever wondered what premultiply is and what it does to an image, then in this video I will show you understand it. Also here I have demonstrated what un-premultiplication is and when to to use it while working on comp.