A capital gain is the amount you get from selling property, like stock, a house, or a mutual fund. For example, if you buy stock for $1,000 and sell it for $1,250, you have capital gain of $250. You don't need to include a capital gain if it's from the sale of your main home you owned for at least 5 years (and the profit is less than $250,000). Report any capital gains noted in Form 1099-DIV, which you should get from some companies, like mutual funds, before the tax filing deadline.
For more information, see IRS Publication 17, chapter 14 or IRS Publication 544.