Open Access matters because it eliminates financial barriers to scholarship.
Open Access means that research, data, and other educational tools are available without intensive copyright restrictions and can be read, downloaded, and shared by anyone.
Open Access expands intellectual curiosity, collaboration, and equity in academia.
Academic publishing is a profit-driven world. Most often, publishers own the rights to the articles present in their journals – not the authors. If you want access, you have to pay by individual article or for cost-prohibitive subscriptions. This structure makes access to scholarship inequitable, slows down intellectual advancement, and puts publicly-funded research behind paywalls.
For more information, check out "Benefits of Open" from PLOS.