Mentions refers to the brand keywords of your product or company that people use on their posts - this can be tracked using specific tools.
Click-through rates for social media refers to the number of followers who click a link on your social post that takes them to your website or blog.
Followers are social media users who have opted to receive your content on their timeline.
Impressions refers to how many times your posts showed up on a timeline or newsfeed. This KPI is calculated individually across platforms.