Capital gains refers to the profit you make when you sell a capital asset — such as shares, mutual fund units, property, gold, or bonds — for more than what you originally paid to acquire it. If you sell for less than your purchase price, that's called a capital loss instead, which in many cases can be used to offset gains elsewhere and reduce your overall tax liability.
How capital gains are taxed depends heavily on two things: what type of asset was sold, and how long you held it before selling, which determines whether it's treated as a short-term or long-term gain. Because the rules and rates differ meaningfully across asset types — equity, debt funds, and property each follow their own logic — it's worth checking the specific holding period and tax rate for the particular asset you're planning to sell.