Citation searching is a method of finding reseach related to an article. This is done by looking at the references included in the reference list of the article (backward citation searching) and looking at who has included that article in their own references (forward citation searching).
Forward citation searching can be done through various websites inlcuding PubMed and Google Scholar:
In PubMed, open the full record for the article and scroll down to the Page Navigation section on the right hand side of the page. In this menu choose "Cited by".
Locate the article you are interested in and underneath the article select "Cited by ##"
Here are some useful resources looking at citation searching
How do I do a citation search? - Open University
What is citation searching - City, University of London
Citation Analysis - University of South Dakota
Guide to citation searching - Brown University