“Open-access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions” (taken from Peter Suber's Open Access Overview).
Open access presents an alternative to traditional academic publishing. Typically, with traditional academic journal articles:
However, with open access:
In short, more people can access the material. The idea is that this benefits authors, as their material is more widely read and has more impact, and that this benefits readers as they have access to more material. As a result, society can build on existing research to develop things everyone can gain from more quickly and more efficiently.
The OA movement has primarily focused on academic journal articles thus far but it is by no means limited to them. All sorts of material, including books, pictures, videos, datasets and more, can be made open access. Non-academic material could be made open access too.
For more information on open access, read Peter Suber's Open Access Overview or watch this video entitled Open Access Explained! which, although it focuses on the sciences, is very useful in general terms.