why we use this code in mvc entity framewirk

Question

// GET: /Student/Create public ActionResult Create() { return View(); } // // POST: /Student/Create [HttpPost] [ValidateAntiForgeryToken] public ActionResult Create(Student student) { if (ModelState.IsValid) { db.Students.Add(student); db.SaveChanges(); return RedirectToAction( "Index" ); } return View(student); } // what is meant by model state.isvalid why we check before to add student. and why we use save changes() method here?