A threshold is a strip across the bottom of a door opening that seals it when the door is closed, so cold (or hot) air does not seep in under the.
Begin by removing your existing threshold from your door. You'll want to first cut the old threshold with a back saw. Then you can dislodge it with a pry bar.
In purchasing new door threshold strips you'll need to know which sizes will best fit the doors where you plan to install new strips. Measure each door separately.