From the Australasian Open Access Strategy Group:
"For scholarly work Open Access means making peer reviewed scholarly manuscripts freely available via the Internet, permitting any user to read, download, copy, distribute, print, search, or link to the full text of these articles, crawl them for indexing, pass them as data to software, or use them for any lawful purpose, without financial, legal or technical barriers other than those inseparable from gaining access to the internet itself."
Articles lodged in a repository are often called 'green' open access and are distinguished from 'gold' open access papers where an author may have paid an article processing charge (APC) to publish the final version of record with a publisher.
Research funders are increasingly mandating that publications from research be made available open access.
Open Access: An Analysis of Publisher Copyright and Licensing Policies in Europe, 2020 - SPARC (Scholarly Publishing and Academic Resources Coalition) Europe - 30 September 2020
Open access publishers: The new players - PLOS One - 5 June 2020
To discover the Article Processing Charges (APCs) that publishers charge - check the journal websites directly. Another option is to use the DOAJ website.