To Sarkar, it means that human beings have a spiritual nature. They have needs that cannot possibly be satisfied by material objects. Human beings all seek unlimited joy, but material objects, being limited, can never offer that. The limited cannot yield the unlimited. Only an infinite entity can satisfy the infinite human hunger for enjoyment.
Spiritual activity is simply a pursuit of the infinite entity.
The way Sarkar puts it, it is clear that spiritual needs are an integral part of human nature. In common parlance, the term 'progress' is associated with technical and scientific advancement, or anything which enhances the comforts of life. Humanity is said to have made tremendous progress today because life seems so much more comfortable these days than it was a few centuries ago. People today can travel fast by automobile and airplanes, whereas only in the last century they were travelling by horse-drawn buggies and bullock carts. If we go back to ancient times, people had to travel on foot.