Engagement is when posts made on social media are seen by other users and have an impact on the content. If your status post on a Facebook profile has received 15 likes, the interaction of that post is that it has received 15 likes.
We should not limit the interaction only to social networks. We can also accept values such as comments made on a blog post and sharing that post by users as interaction. Content with higher engagement will help pages or profiles appear more active, and it is also one of the key elements in reaching more people/followers.
Having accounts with high engagement brings along other elements that will help the account grow even more in a short time. For example, if the posts you make on a Facebook page are liked by your followers, it will mean that those posts will be seen by your followers' friends. Or, when a Tweet you posted on Twitter is RTed, everyone who follows your follower will see that Tweet.