As you probably know Google is continuously changing how it determines what to return when somebody searches, and in what order to put them.
Now it is adding the Google + button. The evidence is that if a few people +1 a page it will rank higher in the search results.
The linked article below gives you more information. If you want to add a plus button simply to your site then one of the two following should work.
If your site is based on WordPress then you can add a free plug-in such as plus one.
The alternative is to use the code that Google can give to you here, you may need a website developer to add this to your site.
Read the article here.
Designed by John Nolan.