Let's assume that you've done some initial research and you have a few articles that are relevant to your research question. You could just use those articles in your review and go spend more time in the lab (or have a beer). If you do that you're missing a crucial step: looking for the context in which those articles were created. Knowing how to follow strands of research forward and back in time - following a citation chain - is an essential skill to have as a researcher.
A citation chain shows the relationship between a specific article (Smith 2002) and the research that cites, or is cited by that article. Research doesn't take place in a vacuum - researchers build on and expand on previous knowledge. This knowledge is interconnected through a citation chain like the one shown below:
As you can see from the diagram, Smith (2002) cited some previous research, and also has been cited by others who are building on that work. Notice that the relationship isn't strictly linear, it's an interconnected web of research.
Citation Chain Diagram from: https://undergraduatesciencelibrarian.org/2010/10/29/diagram-of-the-citation-chain/
Hidden in the articles you've found so far is a wealth of information! The bibliography is a roadmap that shows who our author (Smith 2002) thinks is important in the development of their research. By looking at the references in a bibliography, you can explore the context in which Smith's research takes place. Finding articles in a bibliography is a quick way to find more research on your topic. You can then check the bibliographies of those articles and (perhaps) find more relevant research.
To follow up on items from a bibliography, you'll need to get copies of the articles. You can search for them using our Articles Index & Databases, or Google Scholar. You'll also want to follow those citations forward in time using cited reference searching.