A publication agreement is a legal contract between you & your publisher. Among other things, it determines
You can assign/transfer your copyright to the publisher, but at the same time reserve some specific rights for yourself.
Alternately, you can retain your copyright, and license specific rights to the publisher.
Rights you might want to consider include:
How can you share an article you've published (or plan to publish) in a journal?
Maybe you'll want to post it on your website later on, or share in your discipline's subject repository?