Citation chaining or mining is a research strategy that allows you to pursue a topic both forward and backward in time to find relevant information to use in your research projects. By going backward and forward, you can see how the thinking about an idea or topic has evolved or changed over time.
All databases will list an article's or book's references, or citations. Many times, these are hyperlinked to the full text of that article. This is is called backward chaining because you are looking back at what was written about a topic.
Some databases also tell you which articles cited the article you are reading. This is called forward chaining. Just look for a link or a list that says Cited by.