The invisible like simple refers to a person appreciating a piece of social media content without explicitly using the ‘like’ functionality the social media platform provides.
The terrible irony
In the offline world, the verb ‘to like’ is simply to appreciate. No explicit action is necessary for that appreciation to have value.
In the online world, ‘like’ is taken to be an explicit action… hence the need for an intermediary term for the online world – The invisible like.