MLA format is a way of organizing and presenting your writing. It includes rules about how big the font is, how much space to put in the margins, how to give credit to the sources you used for your research, and more.
When every paper looks exactly the same, you can be sure that the teacher is grading on the paper's content, not the way it looks! MLA formatted papers are also easier to read because they use a clear font and even spacing.
In addition, the style gives you a way to give credit to other people for their words and ideas, so you can avoid plagiarism.
MLA is the Modern Language Association, the organization that invented the style.