The term “capital gain” refers to a profit which is earned on the sale of assets which you already own.
If you buy securities, real estate, currency, precious metal, a car, a bicycle or even a piece of clothing and then sell those assets at a profit, the difference between the price you paid for the assets and the price you sell them for is a capital gain.
The term is often used more specifically to refer to profits earned on investments or on assets which are bought and resold for business purposes.
Money which is earned through means other than the sale of assets you own is referred to as “income”.
Capital gains are normally taxed separately from income, with capital gains tax being levied on capital gains and income tax being levied on income.