I’ll start by echoing the message from other sites that went through beta, voting helps us define the scope of the site and figure out who the experts are. To quote the Vote early, vote often blog post:
Putting voting front and center is very much intentional; it is how good content is voted to the top wrong or incorrect content is voted to the bottom users who consistently provide useful content accrue reputation and are granted more privileges on the site It’s only through voting that a class of editors, closers, and moderators can emerge to help run and govern the site. Voting is how site leadership forms. That’s why the reputation leagues show a breakdown of reputation spectrums.
As someone who has been through private betas before, I can’t stress how important this is now and throughout the site especially in the early days. Every vote up or down will help shape the site and that shaping will be so much amplified in the first few days as the first real questions and answers pour in. Users will look at highly voted questions and answer not only as good information, but as examples of what answers and questions should look like on the site. Users will also go to the meta and see how the community feels about topics and ideas related to running the site because of voting done on the meta. It’s the most important aspect of the site and will be the biggest contributes you can make to it.
Also, please keep in mind when voting that your votes are yours. You don't have to up or down vote something just because others have. Read it, make your decision about the content, and vote accordingly.