1. For a call option, when the option's strike price is below the market price of the underlying asset.
2. For a put option, when the strike price is above the market price of the underlying asset.
Being in the money does not mean you will profit, it just means the option is worth exercising. This is because the option costs money to buy.
Watch: In The Money
In the money means that your stock option is worth money and you can turn around and sell or exercise it. For example, if John buys a call option on ABC stock with a strike price of $12, and the price of the stock is sitting at $15, the option is considered to be in the money. This is because the option gives John the right to buy the stock for $12 but he could immediately sell the stock for $15, a gain of $3. If John paid $3.50 for the call, then he wouldn't actually profit from the total trade, but it is still considered in the money.