If you can’t get to it, then you can’t use it.
If I can’t get to a classroom, I’m going to miss the discussion, the lecture, lose the ability to ask questions, and my learning is very definitely impacted. We make our physical spaces accessible by following design standards that are planned to accommodate all of the bodies and abilities we can imagine in all of the ways we can think of. Physical space design standards are defined by the Americans with Disabilities Act (ADA) and illustrated in very specific ways here. Anything that impacts a person’s movements, from the size of a doorway to the lack of a ramp or elevator, literally prevents their access to learning.
A learner's ability to access a learning space and use the resources in it is the most basic definition of accessibility in learning. That applies whether we're talking brick and mortar buildings or online learning spaces. Our goal as instructors is to communicate the same information to all of our learners, give them space to process it and apply that knowledge through projects, teaching or other learning assignments.
There are a lot of issues that can come up when it comes to access some that will impact different audiences based on their abilities. Are you doing a live video session? Do live video sessions have captions or a transcript? Are colors used to communicate status information that a colorblind user might miss out on? There is a lot to think about but don’t let it be overwhelming. Access issues are solvable, and we have the technology and capacity to do it. And the best part is, better access is beneficial to everyone. That transcript of a video can be reformatted as a QuickStart instruction guide. A text description in place of a color-changing icon means that both a colorblind user and a low vision or blind screen reader user gets detail they would have otherwise missed.