The goal of a thesis statement is to establish focus and make a claim. It's the heart of the essay and is meant to grab the readers attention. Without it the reader won't have a idea about the side/claim you take until they get into it. It's in the introduction and usually is the last sentence. It's needs to be strong and get your point/claim across in a good way. Also to make a good impression on the reader.
Hope this helps!!
