The planning stage of building a tree house is the fun part — the hard part is the execution of your plan. Execution is taking an idea and actually making it happen.
The execution of a plan is when you put it into effect, like the execution on the field of a football team's game plan. It can also mean the style in which a project is carried out, like a ballet's creative execution. Execution can also refer to the death of a person, either a prisoner who's been sentenced to death by a court of law, or a deliberate, targeted murder, especially when it's done for political reasons.