There are plenty of articles online that do a fantastic job explaining the basics of relays. I will complement this article with some links on the reference section.
However, the goal with this article is to provide a bit more information and sometimes present some of the concepts with a different perspective. There are sections with some theoretical concepts that can be intimidating at first. Don't get overwhelm, you can always digest those advance concepts later as you get comfortable with the subject. You can also skip sections and read just the parts that are relevant to you.