Accessibility (often abbreviated to A11y—as in "a" then 11 characters then "y") in Web development means enabling as many people as possible to use Web sites, even when those people's abilities are limited in some way.
For many people, technology makes things easier. For people with disabilities, technology makes things possible. Accessibility means developing content to be as accessible as possible no matter an individual's physical and cognitive abilities and no matter how they access the web.
"The Web is fundamentally designed to work for all people, whatever their hardware, software, language, culture, location, or physical or mental ability. When the Web meets this goal, it is accessible to people with a diverse range of hearing, movement, sight, and cognitive ability." 
The MDN Accessibility Learning Area contains modern, up-to-date tutorials covering accessibility essentials:
- What is accessibility?
- This article starts off the module with a good look at what accessibility actually is — this includes what groups of people we need to consider and why, what tools different people use to interact with the Web, and how we can make accessibility part of our web development workflow.
- HTML: A good basis for accessibility
- A great deal of web content can be made accessible just by making sure the correct HTML elements are used for the correct purpose at all times. This article looks in detail at how HTML can be used to ensure maximum accessibility.
- WAI-ARIA basics
- Accessible multimedia
- Another category of content that can create accessibility problems is multimedia — video, audio, and image content need to be given proper textual alternatives so they can be understood by assistive technologies and their users. This article shows how.
- Mobile accessibility
- With web access on mobile devices being so popular, and popular platforms such as iOS and Android having fully-fledged accessibility tools, it is important to consider the accessibility of your web content on these platforms. This article looks at mobile-specific accessibility considerations.
- Understanding the Web Content Accessibility Guidelines
This set of articles provides quick explanations to help you understand the steps that need to be taken to conform to the recommendations outlined in the W3C Web Content Accessibility Guidelines 2.0 (WCAG 2.0 or just WCAG, for the purposes of this writing).
- Until now, web developers who want to make their styled <div> and <span> based widgets have lacked the proper techniques. Keyboard accessibility is part of the minimum accessibility requirements which a developer should be aware of.
- A collection of articles to learn how to use ARIA to make your HTML documents more accessible.
- Assistive technology (AT) development
- A collection of articles intended for AT developers
- Mobile accessibility checklist
- This document provides a concise checklist of accessibility requirements for mobile app developers.
- Cognitive accessibility
- When creating web content, be aware of how you can ensure that it is accessible to people cognitive impairments.
- Accessibility for seizure disorders
- Some types of visual web content can induce seizures in people with certain brain disorders. Understand the types of content that can be problematic, and find tools and strategies to help you avoid them.