Что означает using в этом коде?

public ActionResult Register(RegisterModel model)
{
    if (ModelState.IsValid)
    {
        User user = null;
        using (UserContext db = new UserContext())
        {
            user = db.Users.FirstOrDefault(u => u.Email == model.Name);
        }
        if(user == null)
        {
            using (UserContext db  = new UserContext())
            {
                db.Users.Add(new User {Email = model.Name, Password = model.Password, Age = model.Age });
                db.SaveChanges();
                user = db.Users.Where(u => u.Email == model.Name && u.Password == model.Password).FirstOrDefault();
            }
            // ecli polzovatel' udachno dabavlen  v bd 
            if (user != null)
            {
                FormsAuthentication.SetAuthCookie(model.Name, true);
                return RedirectToAction("Index", "Home");
            }
        }
        else
        {
            ModelState.AddModelError("", "Pol'zovatel' s takim loginom uzhe sushestvuet");
        }
    }
    return View(model);
}