Open Access refers to materials (usually digital) that are made freely available. It includes most of what you can legally find online without a paywall.
In academia, Open Access usually refers to scholarly articles, textbooks, and eBooks that do not require payment or subscription to read their full text. This is different from materials in library databases, which the library pays for. Often, the author of an Open Access work is the one who pays the publishing fees.
Open Access works are still protected by copyright! Just because something is freely available to see or read, does not mean it is free to use in your own work.