These are not vague ideas, but very specific. The very successful ones can describe their whole dream to the very detail.
It could be their dream house for example, where they can tell you how and building looks like, the architecture, the location of the home, number of rooms, the interior styles of each room, the types of furniture, and the list goes on.
There are others in business who can point directly their dream business - what it does, how big it wish to grow, the concept of the business, etc.
On a smaller scale, there will be times when something crops up, and we just need to make a quick decision, as whether we want to do, and what to do. Again, the statement of knowing what we want is important.
Ive experienced a few friends who just did things because they don't exactly know what they want. Their actions will take a lot of their time and energy and yet not at all bringing them closer to their goal of achieving success. It's a pity because some of them are very talented and ambitious, but have diluted their energy and resources.
Some examples could be going for meetings and seminars which don't add value; or others don't do what they are asked for hence not making actions or decisions complete.
I learnt about this principle and realised that it works for us once we master it. Then, we will have more free time and at the same time achieving our goals - big and small.