The traditional scholarly publishing model used subscriptions to fund publication costs, and only subscribers could read the articles (or you could request through your library, and it might take awhile). The cost of publishing fell on subscribers (usually libraries).
Open Access is about making science more open and accessible. The authors pay an Article Processing Charge [APC] to the journal (after the article is accepted) for handling publishing / keeping the article available.
Some journals are "hybrid" - you can publish the traditional way or pay the APC when your article is accepted.
NIH funding requires authors to publish open access (APC funds can typically be built into the grant) in order to make publicly funded science available.
Don't be taken in by PREDATORY/ Deceptive publishers. These publishers often promise super-fast peer review and almost guaranteed acceptance, and are usually NOT legitimate. These publishers solicit unsuspecting researchers, take their money, and your work is basically lost - not indexed in a database, and not well-regarded. Higher quality publishers usually operate on principles of transparency.
Use these to evaluate journals BEFORE you submit.
Duke University Medical Center Library. Be iNFORMEd Checklist
Identifying Deceptive Publishers: a checklist
Journal Evaluation Tool. Rele, Shilpa; Kennedy, Marie; and Blas, Nataly, "Journal Evaluation Tool" (2017).
UNSURE if a journal is predatory? Ask Frances for help.