Capital gains are the profit from a sale of property or an investment
When you have investments, you are not taxed until you have capital gains. Capital gains are only triggered when you make a sale at a profit.
Long-term capital gains are after you have held your investments for longer than a year. These get preferable tax treatment.
Short-term capital gains are taxed at your current tax rate. Short-term means that you held the investment for less than a year (generally a no-no).