So they'll be a shorter version of this on the main site soon, but I can spell it out at greater length and more informally here. (I'm EFF's International Director, btw)
The short answer is that you're signing your support for a set of 13 principles on the application of human rights to communications surveillance ( see https://necessaryandproportionate.net/ ), that were worked out last year (pre-Snowden, actually) by a coalition of technologists, privacy activists, and legal scholars.
The long answer: We've been using this language to push the idea in international venues and among key lawmakers in various countries that mass surveillance (as well as a bunch of other practices conducted by the NSA and other spooks, including corruption of crypto standards and backdoors) is a violation of existing human rights standards.
This is important internationally because if the NSA gets away with its current behaviour, it'll establish a norm that such surveillance is okay for any government to conduct. We need to push back against that norm.
To do so, diplomats, policymakers and others need language and arguments to back that up.
The Principles give them that language in a familiar context (and we're working together to provide more detailed arguments and other legal guides). It has found favor among politicians, experts and other influential people I think partly because the smarter ones are genuinely worried that pervasive surveillance really could undermine their own societies -- they recognise what it could do in the wrong hands as much as anyone else.
In providing that leverage, it helps to convey that it's not just a bunch of domain experts that think forbidding mass surveillance is a bad idea, but that an increasingly large number of citizens find it abhorrent. That's what the signatures will do. It really makes a difference in this arena, because so few obscure technical documents have hundreds of thousands of supporters :)
I'd encourage anyone who wants to understand better how we're trying to get all governments, not just the US, to craft better surveillance legislation to read the full text of the principles at https://necessaryandproportionate.net/text You can also ask me questions at email@example.com . It's a long haul project, and we're conducting it alongside legal actions in the US and abroad, shoring up and disseminating crypto tools, and other non-policy defences. But it's pretty amazing to get unanimity with hundreds of privacy groups on some basic principles with which to start building proper, 21st century, surveillance law.
1. Legality - privacy restrictions must be prescribed by law.
2. Legitimate Aim - What you want to break privacy for must be to support laws.
3. Necessity: We cannot (reasonably) achieve the aim without breaching privacy.
4. Adequacy: Listening to your phone calls must reasonably allow us to achieve the aim.
5. Proportionality: Don't listen to everyone when only some will do.
6. Competent Judicial Authority: Any breach of privacy must be authorised by Independant, capable judges.
7. Due process: Who follow a clear process
8. User notification: and tell you you are being watched (unless that might hurt the Aim)
9. Transparency: We get to see the metadata on their phone tapping
10. Public oversight: and a few of us get to see everything not just the metadata
11. Integrity of communications and systems: backdoors are not allowed, if you are bugging us legally, you dont need a back door
12: Safeguards for international cooperation: No playing arbitrage with different jurisdictions
13: Safeguards against illegitimate access: and secure your stuff.
I hope those help
But my question is, with the publication/confirmation yesterday of the basis of the US drone assassination program in NSA cellphone spying (see https://firstlook.org/theintercept/article/2014/02/10/the-ns...), will you be incorporating a demand to reign in the NSA because their surveillance is killing innocent people? I think it is a real opportunity to reach out to many other groups and individuals who are opposed to and organizing against the drones both in the US and abroad.
Yesterday, I was this close to putting my name+email there, until I thought "wait, what am I even signing for, either this might actually accomplish something important and I really should know what exactly before I can honestly give my support, or it's a rather meaningless gesture carrying just a vague anti-surveillance sentiment" (given the names and people involved I was pretty sure it was the former, but not knowing what I'd feel disingenuous about just adding my name to a list "just, make it better, or something").
Knowing this now, I signed it right away.