When you follow a citation trail, you are looking for the resources that have either:

(1) been previously published and are referenced within the item you're reading, or

(2) been published after the item you're reading, using it as a reference 

Let's say you are reading an article published in an academic journal in 2012.

The author has referenced several other articles that you would also like to read. Using the article's bibliography, you hunt these down. In a sense, you are following a citation trail back in time. This is called backwards searching.  

As you continue to read the 2012 article, you decide this is a particularly important publication. You would like to see the articles that have been published since 2012 that have referenced it. Using a database, you hunt these articles down. In this case, you are following a citation trail forward in time. This is called forward searching.

Academic research and publication is a conversation that researchers are having over time. 

When you read a published academic work (such as a journal article, book, or book chapter), you are joining this conversation at a particular point in time.

To understand this conversation, you need to collect what has been said before the point where you joined. You also need to collect what has been said after that same point.

Citation trials help you move both backwards and forwards within this conversation!