The internet can "prove" the importance of any random fact. Once everything is important, nothing really is. To get to the important, let's first learn to isolate what's unimportant.
Economy means "frugality in expenditure." We can apply the concept to any limited resource. If you get something done with the LEAST amount of resources, then the leftovers were unimportant.
It's easy to pull out the big guns for every small problem. But find the solution that fits just right is an art. Without understanding this concept, startups sometimes build "all the features under the sun." Todo lists get filled with unproductive activities.
The moment you move toward efficiency, you'll start recognising things that are unimportant. For example, many anecdotes you find in non-fiction are unimportant, because the author usually comes up with them after he finalises his primary message.
In short, what's the least you could do to achieve a result, without comprising the quality of the result.