You can use it or that to answer the question: What does x mean?
It means is a direct, universal or general answer, as found in a dictionary or in your own head. It means is used to express general truths about something. For example, regardless of what a person says about something, it means could be used to contradict a given definition of something in statement form.
That means refers to what a person wrote or said. When speaking, we usually will say "that means" in response to what a person says. In forums, we can say this is speech, as opposed to formal writing in an essay or paper or article.
However, if you provide an answer or response, and then continuing saying something about the thing, you can switch to this, which then refers to what YOU are saying.
Q: What does speech act mean?
A: It or that means a person speaking at a particular time. Sometimes it is a single sentence or it may be more or less than a single sentence in terms of grammar. Also, this means that the act is spoken and not written.
SUMMARY: In the example above, this is used by me to refer to what I have just said.