As you’ve seen, it means that a large body of scientific research is accessible without charge, free on the web and Open Access Helper is a tool, which will help you find these Open Access copies, even if the main publication is still behind a paywall.
Open Access Helper does this by neatly integrating with Safari on macOS and iOS and then interacting with the APIs from unpaywall.org, core.ac.uk and openaccessbutton.org to lookup possible Open Access copies for these documents.
The copy identified, might differ from the published document and thus, whenever we get the Open Access copy via unpaywall.org, the tool will clearly identify the version presented.
You’ll find these labels: