Capital Gains Tax
Capital gains tax is what you owe on the profit when you sell an investment for more than you paid. Assets held over a year get preferential long-term rates (0%, 15%, or 20% depending on income); assets held a year or less are taxed as ordinary income, which is usually higher.
The one-year holding line makes timing matter: selling just after a year can meaningfully cut the tax. Gains inside tax-advantaged accounts (401(k), IRA) aren't taxed as you go, which is a large part of why those accounts compound faster than a taxable brokerage.