Open Access content is freely available to all and grants reuse rights to readers. Journal articles, books/chapters, or other types of scholarly content can all be OA.
Open Access publishing benefits the researcher as well as the reader by:
It has been used now for decades and by some counts, up to 80-90% of research at top-performing universities is already OA in some form.
There are two main forms of Open Access: Green and Gold.
A so-called "hybrid" journal is not quite Green or Gold -- hybrid is when the entire journal is traditionally published, but you as an author can choose to pay an APC to make your individual article OA.
Some researchers and writers worry that Open Access journals are not peer-reviewed or are poor quality. Indeed, some unscrupulous publishers use the pay-to-publish model (Gold OA) to take advantage of researchers eager to get their work published anywhere.
But there are ways to verify the legitimacy of Open Access sources: