Short answer: open access publishing makes scholarly work available online for readers without a paywall. In many cases, it also uses licences that allow wider sharing and reuse.
Open access exists because research locked behind paywalls can be difficult for patients, teachers, independent researchers, journalists, policymakers, and the public to read. If public money helped fund research, many argue that the public should be able to access the results.
How open access works
Some journals are fully open access. Every article is free to read. Some are hybrid, with both paywalled and open access articles. Researchers may also place versions of their papers in institutional repositories, subject repositories, or preprint servers, depending on journal policy and funder rules.
Open access does not always mean free to publish. Many journals charge article processing fees. Those fees may be paid by universities, funders, institutions, or authors. Other open access journals charge no fees and are supported by societies, libraries, grants, or universities.
Open access and quality
Open access is not the same thing as predatory publishing. Many excellent journals are open access. Many questionable journals are also open access because author fees can be exploited by publishers that accept weak papers for money. The business model alone does not tell you whether a journal is trustworthy.
To judge an open access journal, check its editorial board, peer review process, indexing, publisher transparency, fees, corrections policy, and reputation in the field. Open access is about removing barriers to reading. Quality still depends on editorial standards and research integrity.