Make products that can be used by everyone. Traditionally, we think in terms of making things that people with disabilities can use, but in reality this benefits everyone.
That's accessibility. Simple as that.
While it may be simple, it's not easy.
We want it to be easy. And that's what makes it complicated.
It's complicated because there's a conflict between what we expect and what is real.
Accessibility cannot be easy. Just think of the vast spectrum of human abilities and needs we must consider when building products. What helps someone with limited mobility might hinder someone with visual impairments. A design that works perfectly for a person with dyslexia could be challenging for someone with motor control issues.
Then consider all the technical constraints, budget limitations and the need for it to "look good."
Accessibility isn't static. Technology moves faster than what we can keep up with and so do the tools and standards for making accessible products. What worked yesterday might not work tomorrow.
So your challenge is to create a solution that's flexible enough to adapt to changing needs.
And while that sounds simple, it's never easy.