Accessibility refers to whether or not a product or service is usable to those with disabilities. For instance, ramps are a form of wheelchair accessibility, in that they allow wheelchair users to go where they otherwise would not be able to, due to physical barriers. These barriers might be a series of steps, or a street curb without curb ramps.
Accessibility doesn't have to refer to the usability of a physical product, it can extend to things like web sites, software, and apps. As Mudlet is rapidly gaining new accessibility features, the pages linked here are aimed at documenting Mudlet's accessibility features.