What is Open Access?
In simplest terms, Open Access (OA) is a set of principles or concepts that aim to provide academic content at no-cost to the user. Everything online costs some money to someone (costs to create, costs to maintain a website, costs to host/server space), so nothing is "free." In most cases, what we think of as "free" is paid by someone/something that has an interest in users seeing their content. Websites bring in money for profit or simply to stay online through advertisements (ie, Google), subscriptions or memberships (New York Times), by selling goods directly (Amazon), or by donations (Wikipedia).
Open Access scholarship like journal articles, eBooks, reports, and more seek to circumvent the traditional scholarship model of paid access. Traditionally, students and scholars got academic research through personal or institutional (library) subscriptions to individual journals or packages of journals and articles via databases. For the cost of the subscriptions, library users get access to generally credible and reviewed content, all in one place.
Open Access journals and articles are not inherently better or worse than traditional journals behind paywalls. Either can be expertly or poorly written or edited. Few open access journals have the level of prestige and history of the highest levels of traditional journal publishers, but many cost-conscious authors are now shifting their preference to Open Access journals.
Currently, the sciences have a wider breadth of open access journals compared to the humanities.