This is closely linked to the theme of clear intention, but is a simply mental strategy to ensure you don't find yourself about to hit without a sense of purpose.
The word decision derives from the Latin, and literally means to "cut off". It's helpful to think of a true decision as one where you have cut off all other options.
The decision line in golf is an imaginary line you set between you and the ball as you begin your pre-shot routine. You commitment (within clear intention) is not to cross that line until you have made a clear decision on the shot.
It's better to be decided than right
In my experience coaching, far more and far greater errors come about from indecision than poor tactics. The decision line adds a sense of purpose that tends to feed into your body language and supports a confident swing.