A thesis statement is a one or two sentence condensation of your whole essay. A short paragraph which contains/highlights the main argument you are trying to make in the essay as a whole. Similar to an "abstract", but shorter still in length.

Very short essay: --- There have been many attempts to reform English spelling but they have all failed. Noah Webster succeeded in making the U.S. spellings different from British spellings without making it easier. Melville Dewey founded a Spelling Reform Society and eventually began spelling his name "Melvil Dui," but although he was successful in creating a library cataloging system still used to this day, his efforts at spelling reform went nowhere. George Bernard Shaw hoped someone would create a true phonetic alphabetic, and after his death someone offered a prize in his memory, and a phonetic alphabet was designed--but only one book was ever printed in it. It seems as if students of English will have to cope with English spelling for a long time. --- In this very short essay, the first sentence is also the thesis statement. The thesis is "There have been many attempts to reform English spelling but they have all failed."