The Audience overview is found under the Audience tab, and overview.
The key statistic here is the number of users rather then the number of sessions as this is the number of actual people who have visited your site.
The Social Sources are found under the Acquisition tab, under All traffic ->Channels and click on Social.
This then gives you a breakdown of which social channels are driving what traffic to your site.
This shows that in the current period Facebook has been driven more of the social traffic to the site, followed by twitter.
The Site Content is found under the Behavior tab, under Site Content ->All Pages.
This shows which pages are the most popular with your visitors, this is so useful to see what your visitors are interested in on your site and the pages they are going to.
The Site Content is found under the Audience tab, under Mobile ->Overview.
This shows you a break down of which devices your audience are using.
It is essential that your site is mobile friendly too, you can test your site using googles free mobile friendly testing tool.
Page Timings is found under the Behavior tab, under Site Speed ->Page Timings.
Page timings are of great importance if your site is running slow and your pages take over 4 seconds to load google may down rank your site. The main factor with your load speed will be the server your site is hosted on, so using a good host is vital.