Currently, there are over 600 packages that have a different version in sid than they have in sarge. Bugs are closed when they are fixed in sid, bugs aren't reported very often if they are only present in sarge, and not in sid, and sid simply still has more users than sarge.
This situation has a high risk of letting important bugs slip into the sarge release, and that is not good. Therefore, on this site it can be registered for which packages it is important that the new sid packages make it into sarge, and for which packages the newer sid versions are only cosmetic changes of which it doesn't mind very much whether they are in sarge, or of which the important parts are already backported to sarge.
What to judge: you need to judge whether the difference between the current sid version and the current sarge version contains one or more fixes that are important or maybe even release-critical to get into sarge. The three options you have are:
Please be aware that the changelog may not mention every change, so looking at the changelog diff is at best a memory helper. For example, the sid version may feature a new upstream, but the changelog fails to mention a security issue that was fixed with it.
Also, note that the fact that the current sid version itself contains bugs, and is in some ways even a regression, doesn't mean that the change isn't critical or important. If these issues are filed as RC bugs, they will be taken into account.
The second part is for logging in. Please use the same emailadress as you use for Maintainer: or Uploaders: fields, as soon there will be a distinction between judgements of maintainers and judgement of all other people. With 'Request token' you'll be mailed a authentication token (password) so that nobody can spoof judgements, with 'Login' you'll be logged in. Once logged in, you can choose to either logout again, or 'Commit', in which case all currently filled in judgements and reasons are committed.
The last part is the list of packages. This list is updated daily around 05:00 UTC. 'No opinion' can be choosen if you don't want to change the judgement, e.g. when you're not 100% sure. You can even then add text for helping others.