The context in which we use the phrase Open Access refers to papers in journals. We say that a paper (or a book or report) is Open Access if it is easy to find on the internet, and you don’t have to pay money to read it. If you have to pay money to see it, we say that the paper is “behind a paywall.”
The journal Biodiversity Observations is Open Access. That means that you can download any paper in the journal without payment. Some of the first Open Access journals were developed by the Public Library of Science (PLoS). PLoS developed the logo which is now used universally to indicate that a paper is “free to read”.
The research output of the BDI is listed here. Papers which are Open Access have these words just under the title of the article., and if you click on Open Access, the link embedded there takes you directly to the paper itself.
There are many wrinkles on the concept Open Access, and there is an article in Wikipedia which defines all the multiple flavours of Open Access.