A capital gain is the amount you get from selling property, like stock or a house. 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).
For more information see IRS Pub 17, ch. 14, pg. 104 or IRS Pub 544.