A holding gain is a gain of value that incurred after holding an asset. This gain happens over a period of time and the owner can sell it at any time in exchange for cash or other assets.
For example, an individual person bought a land worth Rs.5 Lakhs 10 years ago and the same land carries a value of Rs.20 Lakhs. So, the owner has a holding gain of Rs.15 Lakhs. The value of an asset depends on its demand over a period of time.