Your Apple iPhone and iPad are built so you cannot modify the operating system (iOS), nor can you download apps from anywhere other than the official Apple App Store. The term “jailbreak” has been given to the process used to modify iOS and to remove download restrictions.
When you jailbreak your iPhone or iPad, you have the ability to freely modify your device’s look and feel. You can also download any apps of your choosing. However, it is important to think before you jailbreak your device. The practice is considered dangerous because it will make your device vulnerable to malware and other viruses found in third party apps that Apple screens out of its App Store. These viruses can steal the private information you store on your phone, and make you vulnerable to theft of your financial or identity details.
For more information on avoiding viruses, see “How can I avoid getting a mobile virus?”