Google+ for Mac is an unofficial app for Mac desktops that allows you to access Google+ directly from your Dock.
Google+ for Mac requires Google Chrome and uses the web interface of Google+ to allow easier access from your Mac. The developer emphasizes that Google+ for Mac is a temporary solution until he can create a native desktop app. Note it is not based on a Google+ API - it is just a ported browser.
Google+ for Mac doesn't therefore do anything revolutionary. In fact, it merely puts an icon in your Dock that when clicked, opens your Google+ profile in Chrome. Navigating to external sites is impossible because clicking on them leads you nowhere.
You can also achieve much of the same effect as Google+ for Mac with Fluid.app but the developer promises Google+ for Mac will go far beyond what you can achieve with Fluid in the near future.
Google+ for Mac is for all those Google+ addicts that want instant access of Google+ from their dock.