What is a validator?
The translation platform is open for any wordpress.com user to suggest a new translation. When they do, that leaves those strings with a status of "suggested". In order to transform them into "approved" strings (which are the only ones that get deployed), a validator needs to accept (or reject) those suggestions.
In addition to these permissions, a validator can also upload external files (typically, but not only, the .org translations) and discard warnings.
A string translated by a validator is automatically approved (but will still generate all applicable warnings)
A few words on requesting a "validator" profile for your language, on http://translate.wordpress.com:
There is no technical limit on how many users can be validators, however, translation communities should only have a couple of validators so that we can keep track of who to talk to should the need arise. I suggest that you work it out among each other who the two or three should be who can validate, and have all others who want to collaborate organized in these forums. Feel free to create a new thread for your locale, but try to use English as the language for discussion as the items you debate might be relevant to others.
I would also like to strongly encourage all translators (particularly validators) to get in touch with their respective .org translation community, so that consistency can be maintained across versions.