performance indicator or key performance indicator (KPI) is a term used to identify a type of performance measurement. Companies may use KPIs to evaluate the success of a particular activity in which it is engaged. 

Why do you need to understand KPIs?

In social media, key performance indicators can help marketers identify the success of specific activities, campaigns, or pieces of content on Facebook, Twitter, or another social network. These are not to be confused with business outcomes. For example, a Like might indicate an endorsement of a piece of content and therefore is a good performance indicator. However, unless that Like can be attributed to the achievement of a specific business goal - ie.: a sale, a lead - it is not a business outcome in itself.