Think of it as a like button for websites: if you find something cool you can recommend it to your friends who may receive these via their search results, personalizing their searches. More information in the explanation video.
Even though it’s located on the right sidebar it works on a per address basis: subpages – such as standalone posts – can be plus-oned independently. Additionally, it seems to have none of the horrid performance issues of the Facebook like button.
+1 if you like :]