Samantha T. answered • 01/10/23
High School English and Public Speaking Tutor
It means whoever is saying this thinks that what they are doing is very interesting. "Doing this" is a gerundive phrase, basically a verb in the -ing form being used as a noun--the act of "doing" becomes the thing that is being talked about in the sentence, the subject of the sentence. "Interests" is the action, the main verb of the sentence. To say that something interests you is to say that you find it interesting/it is interesting for you. "Greatly" basically means "to a large extent" or "very much." "Greatly" is an adverb, meaning it modifies/describes a verb, adjective, or adverb. Here, it's modifying the verb "interests," making it clear that whatever the speaker of the sentence is "doing," it "interests" them very much.
