There's a new Blog post up as of March 26th explaining ESO's Veteran Content. It's a system for people who reach level cap at 50 to continue to have new experiences and explore new area's once at level cap. Once you hit 50 you continue to "level" by earning Veteran Ranks.
For PvE Veteran Rank zones are the core. It's basically being able to go to the lands of a second, and eventually third alliance and experience their content. It does scale (via phasing) and is "harder" than the content would have been had you played through it on a new character in that Alliance. At least that's my understanding. It's a way for you to progress one character and not worry about having to make alts to explore the other Alliances.
There's lots of other information in the blog post, and you can follow this link to read all about it for yourself!