An object will stay at rest, and an object will stay in motion at a constant velocity, unless acted upon by an unbalanced force.
2 Balanced Forces with an Object staying at Rest
An object will stay at rest when 2 opposite forces are balanced. For example in the picture above the elephant on the left is trying to pull the log back while the elephant on the right is trying to push the log forward. Since both elephants are pulling and pushing at 200 N force, the forces in a way cancel each other out, and because both of the elephants are pushing and pulling at 200 N force, the net force is 0 N, making the log still. In conclusion the log in the middle is at rest because the 2 elephants are both pulling and pushing with the same amount of force.
2 Unbalanced Forces with an Object Accelerating
In order for the skateboard to accelerate forward the skater must push his foot against the ground to propel forward, this movement creates an unbalanced force. In this image the board moves forward because the the force of the skaters foot pushing off the ground creates a bigger force than that of the force of the air resistance and friction acting against the skater's push. The push of the skater's foot is the force of 120 N and the air resistance and friction is the force of 100 N, this means that the net force is 20 N forward. In conclusion the skateboard is moving forward because the forces acting on the board are unbalanced.