Paypal IPN method can be used to inform your website about a successful transaction when the transaction status changes. Some transactions take longer time and some users close the tab, before the user is redirected back to your website. In such cases your website has no clue about the transaction unless you are using IPN. When the transaction is completed, PayPal sends a message about the transaction to the website without customer interaction. When your website has been informed about the transaction it will store the transaction details and send an email to the customer.
You can enable IPN like this:
We suggest you to enable both IPN and PDT. PDT sends the transaction data immediately when the user is redirected back to your website, IPN serves as a fallback if something goes wrong.