✅ I need help understanding ASP.NET savechanges()

[HttpPost]
public IActionResult Edit(Contact contact)
{
    if (ModelState.IsValid)
    {
        if (contact.ContactId == 0)
        {
            contact.DateAdded = DateTime.Now;
            context.Contacts.Add(contact);
        }
        else
        {
            context.Contacts.Update(contact);
        }

        context.SaveChanges(); // Writes changes to database

        return RedirectToAction("Index", "Home");
    }
    else
    {
        return View(contact);
    }
}