The guiding ideal behind UDL is that providing students with multiple means of engaging in assignments and activities that interest them will satisfy their personal and professional needs for learning and at the same time meet the course objectives.
Excellent information on the definition and general concept of UDL is available from the National Center on UDL at http://www.udlcenter.org/aboutudl/whatisudl.
A very brief video introduction to UDL is UDL: Universal Design for Learning at https://www.youtube.com/watch?v=FSHflViWRCQ.
Another good video introduction, produced by the Center for Applied Special Technology (CAST), is UDL at a Glance at https://www.youtube.com/watch?v=bDvKnY0g6e4.