For example, I would say "Why is this thing popular?" And someone else responds with "Because it is popular." What is that type of answer called? I've heard the term before but I can't remember. Another example is "How do I operate this machine?" Response, "By operating it."
Are you thinking of a "tautology"?
1a : needless repetition of an idea, statement, or word
b : an instance of tautology
2 : a tautological statement
It can be a case of a nonanswer, also written hyphenated:
1. an answer or reply that is inadequate or unsatisfactory
Collins English Dictionary
a response that fails to address the subject of a question : an uninformative or unsatisfactory answer.
2.An answer that does not deserve to be called an answer; an inadequate or evasive answer.
Oxford Living Dictionaries
I am thinking it can be described in other ways, if I think of any I'll add to this post.
Addition: Thinking in logical terms, I believe it's a case of circular reasoning:
Academic Douglas Walton used the following example of a fallacious circular argument:
Wellington is in New Zealand. Therefore, Wellington is in New Zealand.
Also in the same article on circular reasoning:
'Whatever is less dense than water will float, because whatever is less dense than water will float' sounds stupid, but 'Whatever is less dense than water will float, because such objects won't sink in water' might pass.
I think the point being made in the above quote is that both examples are equally circular in reasoning, however the second one doesn't sound as ridiculous as the first. The second example gives the reason "because such objects won't sink in water". Pronoun "such" is referring to "whatever is less dense than water. In essence it's basically saying the same thing as the first, except instead of "will float" it says "won't sink".
It may also be an example of begging the question:
Begging the question is a logical fallacy that occurs when an argument's premises assume the truth of the conclusion, instead of supporting it. It is a type of circular reasoning and an informal fallacy.
Begging the question