Willpower is the inner strength that enables you to make decisions and carry them out. It gives you the strength to take action and perform tasks and plans, despite inner resistance, discomfort, laziness or difficulties.
When there is willpower, there is firmness, decisiveness, determination, assertiveness, resolution, persistence, and the power of pushing yourself towards goals and achievements.
With this skill well-developed, you are in a better position to overcome procrastination and laziness, focus on what you are doing, and avoid unhealthy or unreasonable temptations.
Well, that is a dictionary definition, but it is, of course, much more nuanced than just that.
We all have willpower, and we use at least some of it at least some of the time, quite often without being aware of it. But it is only when you have to make a conscious effort to achieve something that you can call that effort'willpower'. While the effort involved can be seen as negative, the outcome is nearly always positive - which is what makes it worthwhile.
There are certain strategies that are helpful to involve. Decide on what your goal actually is. Being too vague about the end result makes it more likely that your resolve will waver. Try and stay calm and positive during the process - after all, you will want your energy to be focussed on what you want to achieve rather than used up fighting yourself.
In terms of goals, decide on just one thing to be achieved at a time. Some targets are complementary: getting fit and eating well often go hand in hand. But while doing one, don't think about the other. Just focus on the job in hand. At the same time, allow yourself flexibility. There can be several paths to the same destination.
If you can, get the support and encouragement of others. It's perhaps inevitable that you will doubt yourself from time to time. Being told that you are doing the right thing, and that you are doing it well does wonders in strengthening willpower. Gadgets and technology can be your friend too. Seeing your results in gadgets and apps can be very empowering.