Engagement Rate refers to the percentage of people who engage with your social media content - either by likes, comments, or shares.
Mentions refers to the brand keywords of your product or company that people use on their posts - this can be tracked using specific tools.
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.