Discipline - choosing what you want most over what you want now. Easier said than done, right?
Most decisions we're faced with have one of two results. One decision will lead to a short-term gain, while another will lead to a long-term gain. Although these decisions mostly have small consequences like watching Netflix over going to the gym, those decisions can compound and have horrible effects.
Choosing to skip the gym once and binge Netflix isn't the end of the world. However, what often happens is that one decision leads to another of the same, and another of the same, and another of the same. One month down the road, you realize you haven't been to the gym in a whole month.
To combat this, it's helpful to make the decision for what you want most, over what you want now. You're feelings will always tell you to take the easy way out.