It just means that a thesis statement is a statement.
The repetition is for emphasis; it helps us to keep in mind that a thesis statement is not a question. Your thesis statement will be the answer to the question, an answer that you will defend and explain in your essay.
But it must be one sentence, not two or more sentences. A declarative sentence is simply a sentence that makes a statement rather than asking a question or making a command.
If you can't express the main point of your essay in one sentence, your essay probably doesn't have one point; it probably has two. It is really saying the same thing twice to say that a thesis statement is a declarative sentence.
You may often start work on your essay with a question in mind. Different essays will have different purposes, depending on your message and your audience.
If you are writing about a topic that your readers know very little about, you will write differently than you would if you were writing about a topic about which your readers were well informed.
I do not mean by a thesis statement something that you necessarily write before writing the essay.
I will often ask you to write a "trial thesis statement" before submitting a draft of your essay; the term "trial" means that this is not a thesis statement you are committed to.
to write up something the write up on someone writing someone/something off write in on something write against something write back to someone write something…
An acrostic poem is like this H-Write something here A-Write something here R-Write something here R-Write something here I-Write something here E-Write something here T-Write something here T-Write something here U-Write something here B-Write something here M-Write something here A-Write…