a) You can reduce your drag by slipping through the water more easily. You'd achieve this through a better body position and better streamlining.
b) You can increase your propulsion for the same level of physical effort. You'd achieve this by developing your catch, pull, rhythm and timing.
Both of these would increase your overall swimming efficiency. Increased efficiency means you go faster for the same effort, or the same speed for less effort.
So which is more important? What should be my priority?
Alternatively if your drag is very high working on your propulsion will only bring you smalls gains in speed. Instead, working on improved streamlining and body position will boost your efficiency and so speed.
If you have access to a coach, ask them to watch you swim and tell you which you should be working on. If you don't have a coach, next week on Feel For The Water we'll be posting some techniques you can use to determine which is a priority for you. It's one of the most useful facts to understand about your swimming.