Pearling is where you examine the reference lists of relevant articles, this may help you discover resources that did not show up in your database searches. You can then use an Internet search to help you locate these resources - remember that they may not have been published online!
Cooke, Smith & Booth (2012) suggests that this approach is similar to snowball sampling, leading the researcher from one study to another until all the leads are exhausted - this is referred to as 'pearl growing' or 'pearling'
Cooke, A., Smith, D., & Booth, A. (2012). Beyond PICOL the SPIDER tool for qualitative evidence synthesis. Qualitative Health Research 22(10), 1435-1443. doi:10.1177/1049732312452938
This is the opposite approach to pearling, in this approach, you're looking at the resources that have cited the resources you have identified.
Two good databases to start this approach with are
Many of the databases that you search in may also include citation information, so check these as well.