Why is perfect tense called "perfect"?
It comes from Latin.
In Latin, "perfect" means completed.
An example of the perfect tense is:
I have read the book.
The action is complete in regards to present.
An example of the imperfect tense is:
I was reading the book when the phone rang.
The action here is incomplete. That does not mean that the speaker did not finish the book. It just means that at the time the speaker is describing, the action was not completed.