Jailbreaking your iPhone removes some of Apple’s restrictions from the device and allows it to be modified in ways that the manufacturer did not intend. This means that you can download apps that aren’t available on the App Store and customize the device in ways that would normally not be allowed.
While this may sound tempting, the biggest risk of Jailbreaking is that it also removes the security restrictions put in place by Apple in order to protect your device and makes it more vulnerable to cyber attacks. Jailbreaking your iPhone will also void its warranty.