This is done through our blueprinting system. This is a way to visually script what you would like to achieve through code in your game. So, the link you provided, points toward a blueprint that is created to check and see if the left mouse button is down and then set a value for a velocity associated with this blueprint. I believe following some beginning tutorials on Blueprints as well Animation within UE4 would greatly benefit you.
Link to Blueprints :
Link to Animation Blueprints :
After familiarizing yourself with Blueprints and then Animation blueprints you should have a better understanding of how these systems work together and how you can achieve what you want with your project.
If you are still lost here is a YouTube tutorial that covers exactly the blueprint that you linked in your comment.
Sometimes nodes do not need certain inputs. For instance you are checking to see if the animation is valid and then setting a speed and velocity to the other cast. This cast may not need that input.