It's a neat piece of science art, and it also tells us something interesting. The arrows show us that the force on the skateboard is constantly changing, both in magnitude as well as in direction. Now the force of gravity obviously isn't changing, so the reason that these force arrows are shrinking and growing and tumbling around is that the skater is changing how their feet pushes and pulls against the board. By applying a variable force that changes both in strength and direction, they're steering the board.
To summarize, a skateboarder's feet need to do two things successfully to complete an ollie. They need to provide a changing force to move the board correctly (so that the combined force of gravity and the skater's feet add up to the green arrows above), and they need to provide different amounts of force with each foot (shown by the red and blue arrows above) to steer and turn the board into the right orientation.
Smile and wave – Feel free to look around at all the other people at the skatepark that stood by and marveled at your new-found accomplishments (all one of them). I’m sure by this point you’ll be picking up roses that are being thrown at your feet, and taking a well-deserved bow as they begin to chant your name. Take it all in, thank the crowd for their undying affection, and then go get a snow-cone. You deserve it, champ.