Open access can have a wide range of definitions, but generally it refers to work that contains the following criteria:
Open access serves as an alternative to the more common "closed" model of access where creative works are blocked behind restrictions and/or financial barriers. Closed access means only a select few people (subscribers and purchasers, primarily) have access to the work. Under open access, everybody has access to the work and can engage with it from anywhere. And even in open access, necessitated by the Creative Commons license that is attached to the work, the original creator must always be credited. As such, there is never a scenario in which a person does not receive credit for their work, unless they willingly dedicate to the public domain (and even then, some reusers still like to credit the original author!).
Creative Commons licenses are geared directly towards open access. Even at the most restrictive license, BY-NC-ND, others are still allowed to share, read, and freely repost the work as long as it remains unmodified and is not used for any commercial purposes.
This means that any institution that adopts Creative Commons licensing is contributing to a wider network of readily accessible works. While each institution has the freedom to define what constitutes open access to them and may require one of the more restrictive licenses for work produced within the institution, the commonality between all of these institutions and licenses is that access is not restricted. People are able to read and engage with any of the works instead of having to pay for articles, journal subscriptions, or databases.
As such, Creative Commons licenses help ensure that artistic and scientific progress is not locked behind barriers while still ensuring that creators and their works are protected and not used in unintended ways. Under the closed model of access, important works may go unnoticed or may not contribute as deeply to future research as they should because it could be inaccessible to researchers who do not have the budget for the work. Or important pieces of public health research may not be available to the communities who would benefit from that information. Using Creative Commons licenses ensures that these works are all able to those they need to reach.
Open Access - Wikipedia
The Fundamentals of Open Access and Open Research - Springer Nature
Open Access - SPARC