A capital gain is the profit realised when you sell an asset for more than it cost you. Until you sell, a rise in value is an unrealised gain — real on paper, but not yet money.
The distinction matters because tax usually applies at the moment of sale, not while the value grows. That timing is why frequent trading can quietly underperform simply holding: each sale can trigger a tax event that removes money which would otherwise have kept compounding. Rules, rates and exemptions vary substantially by country.