The aim of the citation search (also called snowballing) is to find other relevant publications that are included in the reference list of the seed article (backward citation search) or that cite the seed article itself (forward citation search).
These are very efficient ways of increasing your publication sample.
Wohlin (2014) presents a schematic overview of the citation search process (see below):
Web of Science
Cited references = backward citation search
Citations = forward citation search
Google Scholar
Cited by = forward citation search