A quote you likely hear a lot is "It doesn't matter what you know; It's WHO you know." Well, when you compare the "what" and "who" it is generally true that the "who" is more important. After all, if you don't know who to sell to then it doesn't matter what you build or do. However, something that is implicit in that quote is that you know how to execute.
Execution is probably the most important thing to succeed in both life and business. No matter how much you know, if you can't execute an idea to completion you will never succeed.
If you know the material but can't execute to complete that mid-term paper, you will fail.
If you are the smartest person in your profession but can't execute to complete that very important project for your client, you will fail.
If you do everything everyone asks all the time, you likely wont have the time to see any particular project to completion.
Execution is about commitment, dedication and perseverance to stay motivated through the difficult times so you can reap the rewards of the success that follows completing that project on time, within budget, while overall meeting the clients expectations.
Failure to execute is executing failure very successfully.