If your door threshold is creating a gap that lets in drafts, there are a few things you can do to fill the gap and keep your home warm. One option is to use weather stripping. You can find this at most hardware stores and it comes in a variety of different widths. Simply cut it to size and attach it to the bottom of your door with adhesive. Another option is to use caulk. Again, simply apply it to the bottom of the door where the gap is located. Be sure to smooth it out so that it creates a seal when the door is closed.