Question

what exactly is a thesis

Answer 1

It is the last line in your introduction, it says what you are going to talk about

Answer 2

a proposition stated or put forward for consideration, especially one to be discussed and proved or to be maintained against objections: He vigorously defended his thesis on the causes of war

Answer 3

a sentence that sums up the key points of the topic that you're writing about in the introduction paragraph

Answer 4

The thesis is the one sentence that tells the reader what they are about to read.
For example:
Prompt: Explain what is your favorite fruit and use supporting details to support your topic.
So your thesis would look something like" Apples are my favorite fruit because theyre red, juicy and delicious.