In most jurisdictions, if you buy a stock for $100 and sell it for $150, the $50 profit is a 'capital gain' and is subject to taxation.
The critical distinction for investors is the holding period length. Short-term capital gains (assets held for less than one year) are usually taxed at your standard, elevated ordinary income tax rate. Long-term capital gains (assets held longer than one year) receive a tremendously favored, lower tax rate, incentivizing long-term investment.
In the context of RSU vesting or actively transitioning a portfolio, optimizing to avoid short-term capital gains is one of the easiest ways for high-income earners to preserve substantial wealth. The 'Hold vs Sell' projections on StressTest.pro dynamically account for tax frictions when liquidating positions.