The deal is a voucher for $3 toward an Amazon MP3 purchase. It's limited to U.S.-based Amazon.com customers, and cannot be used on any other Amazon.com merchandise. Notably, you've got one week to use it: the voucher expires on July 12, 2012.
To grab the deal, you login to your AmazonLocal account (you do have one, don't you) and search for the deal. Assuming you have signed up for daily deal emails, you've probably got an email in your inbox with the deal link.
If not, for us this link worked, but it's localized for our account. It's unclear if it will take you to the correct deal if you're in another region, but we'd guess there's a high probability it will, since this deal appears national (we've seen it in other regions).
The voucher is available for purchase through the end of the day; there doesn't appear to be a specific limitation in terms of vouchers sold listed on the site. The voucher is can be used immediately after "purchase." It can be used toward multiple MP3 song purchases (many current hit songs are available for $1.29 at Amazon MP3).
Amazon MP3 uses the standard MP3 format; all songs are sold without DRM (digital rights management, AKA copy protection). You can play them on any compatible device, including iDevices, and can import them into your iTunes library, as well.
Any balance unused after July 12 will be lost.