Ever have a group of 12 people who want to go on a raid? Now you can. In fact any number of people between 10-25 can go on the raid and the raid difficulty will fluctuate on the fly for whatever size group you have – that is amazing and quite powerful. The Flex Raid is meant to gear more toward those social small groups who have had difficulty progressing and also gives them an opportunity to be more flexible in the size or raid they have.
http://www.blizzpro.com/blog/comments/patch-5-4-preview-flex-raiding