While Open Access itself is rather easy to understand, there are a few related terms worth knowing:
Green OA is when some form of an article originally published in a closed journal is then later deposited in an open access repository, for instance when CCA faculty deposit research in the Faculty Research collection in VAULT.
Gold OA is when an article is published in a journal that is itself open.
A paywall is any barrier to reading a publication online. When you click a link to the New York Times and are prompted to purchase a subscription, that is a paywall just as many academic journals have paywalls.
Luckily, the CCA Libraries purchase subscriptions to sets of academic journals, which you can find in our list of databases. While you can use these journals for free, the libraries pay to maintain access to them. People outside the college cannot use them (that's why you must sign in when using the databases from off campus).
Many studies have shown that open access publications receive more citations than closed access ones. Making your research OA will increase not only the audience that can read it but also the academics who can build upon it and thereby increase your own profile.
Journal articles stuck behind paywalled links feel like an anachronism in the Internet age. If you want to email or tweet an article link to a colleague, it's much easier to do so when the work is accessible at a public URL. The fact that researchers have resorted to begging for articles via Twitter hashtags shows how important being able to share openly is.
This is the big one: scholarly publishing as we know it is not sustainable. Libraries, include CCA's, spend increasingly large proportions of their budgets subscribing to paywalled journals. The price of journal subscriptions far outpaces inflation, and certainly the ability of libraries to pay. If Harvard cannot afford the scholarly journals they need, CCA is in an even worse position. Open Access is one of the few avenues out of this predicament: by making publications free and easy to access, pressure is lifted from library budgets since we can rely upon open access resources.
Example from the University of Pittsburgh: the darker blue line, far above inflation (black line), is journal subscriptions.