Hand-searching involves looking through the contents pages of journals, conference proceedings and abstracts. The purpose of hand-searching is to identify articles, which have not yet been included in databases or have not been catalogued or indexed.
You can also find additional studies by looking through the references of papers that you have already identified in your review. Many databases will automatically display the references that a paper has used in their work, with relevant links to access the references. Some databases, like SCCOPUS or Web of Science, will also contain an option for 'Citied by,' which will show you the papers that have cited them since they were published.