Some federally-funded agencies, notably the National Institutes of Health (NIH), require that published research be made publicly available through designated repositories or platforms (in the case of the NIH, this means PubMed Central). Such repositories are referred to as "public access" repositories since they meet the specific guidelines of those federally-funded agencies.
"Open Access" refers, more generally, to digital information that is online, free of charge, and free, in varying degrees, of most copyright and licensing restrictions. Open Access resources typically permit users to download, copy, print, display, distribute, search, index, and link to the information. Open Access resources remain the intellectual property of their creators, who have attribution rights as well as control over the integrity of their work.
Please note that while "Open Access" resources are free and open to the public, they may not meet the "Public Access" requirements of your particular funding agency. See the grant guidelines of your specific funder to determine what, if any, requirements or repositories are mandated.