Cydia is a third party AppStore which is a substitution for Apple App Store. Cydia allows you to manage your third party applications. If you want to download fully working Cydia to your device, then you have to do some additional process called Jailbreak using another 3rd party tool. But jailbreak tool not updated for latest iOS versions. So Cydia Pro is the only way to Download Cydia to your devices. Then refer the following guidelines to Download & Install Cydia.