Answer by Jaap Kreijkamp

function Go() {
    horizontal = Input.GetAxis("Horizontal") * speed * Time.deltaTime;
    vertical = Input.GetAxis("Vertical") * speed * Time.deltaTime;

    var pos : Vector3 = transform.position;
    pos.x = Mathf.Clamp(pos.x + horizontal, -7, 7);
    pos.y = Mathf.Clamp(pos.y + vertical, -7, 7);
    transform.position = pos;
}

The link Lipis gave in comment indeed explains the Clamp function. Basically it keeps the value within a certain range so with Mathf.Clamp(value, -7, 7), if value is less than -7, function returns -7, bigger than 7, function returns 7, otherwise it just returns value.

So by making sure we never set the value outside the allowed range we won't get jittering.