In online marketing, analytics are insights and information from the systematic analysis of website/social media account data and statistics.
Why do you need to understand Analytics?
In social media, analytics tell the brand who the people in their social network are, what types of content they enjoy most, and more. Analytics allow companies to test different content formats and styles, posting times, and other metrics, to see what actually works for the brand. See this example from Facebook's analytics program, called Insights: