Overview: Google Scholar searches across academic disciplines and resources for scholarly literature. Although its search algorithm (the "page rank" engine thing that drives the search) is uniquely Google, Google Scholar in fact resembles other academic or scholarly research databases (search tools) that allow the researcher to search through a very large index to scholarly material from a single platform.
Some of the academic resources you discover using Google Scholar will be immediately available to all -- We call these resources "open access." Open access articles are posted online and made freely available (usually because an author has reserved the right to publish her work in an open repository or online platform without restriction). Quite often, open access articles are also published in academic journals -- So the same article may well end up being available from completely different online platforms.
When looking at a Google Scholar search result, if you see a ".PDF" link to the right of an article, that generally indicates the article is available open access.
Much of what you discover using Google Scholar will not be available open access. These articles are often published in journals that are only available online or in print (paper) at subscribing libraries.