Accessibility is the idea that software (among other things) should be designed to be usable and, as much as possible, convenient to use for all users, regardless of skill level, familiarity, disability, to engage with the software.
Remember that accessibility is a moving target, you may have a limited view of what accessibility means. For example increased font sizes does not help someone who needs the colours inverted.
Start by reading what is accessibility Read the first 2 sections So what is accessibility? and What kinds of disabilty are we looking at? As you are reading keep these questions in mind:
Then have a look at firefox accessiblity checker in action.