This great video by CAST explains the principals of UDL perfectly. Their website offers so many resources to make education accesible to all learners.
Universal Design for Learning (UDL) is a framework that addresses the primary barrier to fostering expert learners within instructional environments: inflexible, “one-size-fits-all” curricula. It is inflexible curricula that raise unintentional barriers to learning. Learners who are “in the margins”, such as learners who are gifted and talented or have disabilities, are particularly vulnerable. However, even learners who are identified as “average” may not have their learning needs met due to poor curricular design.