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)?
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:
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. https://doi.org/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 feature has been retired by Web of Science, it is no longer possible to create a citation map.]
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.
Scopus is a multidisciplinary abstract and citation database of peer-reviewed literature (journals and books). The search results also contain links to preprints (2017 onwards) and research data.
Web of Science is a citation database, covering academic journals worldwide in the sciences, social sciences, arts, and humanities, and conference proceedings. It provides cover-to-cover indexing, from 1975 till present.