To keep it simple, Cydia is Apple's iTunes app store substitute for Apple handheld devices including iPhone, iPad and iPod Touch, which offers tons of apps, games and other stuff. The prime advantage of having Cydia on your iOS running devices is majority of Cydia store apps comes for free, which is very unlikely in Apple iTunes store and that might be one of the major reasons behind jailbreaking iPhone devices.
However, after releasing iOS 8.4.1, Apple has made it difficult to release the jailbreak solution and since all the known Jailbreakers are tight-lipped about the iOS 9 or iOS 9.0.1 Jailbreak, the only option for you is to either downgrade your iPhone 6, iPhone 5s, iPhone 5 or iPhone 4 to iOS 8.4.1 and wait for the iOS 8.4.1 jailbreak to arrive or upgrade your device to iOS 9.0.1 and enjoy the performance improvement along with the legacy of Cydia through the below written guide.
- Open Safari browser app on your iPhone and navigate to Http://9cydia.weebly.com
- You will get a number of download options, just tap on one.
- You'll see a popup prompting "Press install to download your app." Press install button.
- A new pop up will appear prompting "Open this page in App Store." Tap on the open button.
- In iTunes it will show you the Cydia background app namely Gett. Tap on Get button to download it.
- Open the app and use it for five minutes. It will take you through an animated tutorial and so on.
- After five minutes of using the app, navigate to Settings>>TouchID and Passcode and turn off both the Touch ID and passcode option.
- Now turn off your device and switch it on after 10 seconds and volia, you will find the Cydia icon on your iPhone.