You can use the reference list of an article or book to find related literature (snowball method). Citation databases Scopus and Web of Science also offer the option to follow the chain of references in the other direction: which articles have cited a particular article or book (cited reference search)?
Click on the picture to enlarge it.
Example of a Citation Map, created in Web of Science. On the left hand you find the reference list of the article in the middle (Zwaan, R. A. (2014). Embodiment and language comprehension: Reframing the discussion. Trends in Cognitive Sciences,18(5), 229-234. doi:10.1016/j.tics.2014.02.008) and on the right hand the articles citing Zwaan's article (at the time this picture was made, the article has more citations now). [Unfortunately, the Citation Map features has been retired by Web of Science, it is no longer possible to create a citation map.]
When you have a list of publications, for example after performing a topic search in Scopus or Web of Science, this citation information helps you to identify key publications (as measured by the number of citations received) and other potentially relevant publications:
With the handout Citation information in Web of Science (pdf), you can find different types of citation information in Web of Science. You can use this citation information to find additional literature and to select literature, for example key publications or 'hot papers'.
This handout is based on Web of Science. Scopus offers comparable tools. In the blogpost, Five steps to creating a citation overview in Scopus, Scopus explains how you can use their tools.