According to Ron Mace, founder and program director for the Center for Universal Design at NCSU, universal design "is the design of products and environments to be usable by all people, to the greatest extent possible, without the need for adaptation or specialized design". Universal Design for Learning (UDL) is the application of this principle to education.
The National Center on Universal Design for Learning provides an overview on how to create curricula that give all individuals equal opportunities to learn. Many of these principles apply to both students with impairments as well as students with different learning strategies, so creating effective alternative content will benefit all of your students, not just students with an accommodation.
The National Center on UDL also provides a set of guidelines and a framework for creating content that meets the needs of all learners. There are also extensive examples and resources for creating materials that follow these guidelines.