Cydia is a third-party app store that is commonly used on jailbroken iPhones as a way to download unofficial apps.
The quickest way to find Cydia on an iPhone is to swipe down from the middle of the screen to bring up search and type “Cydia”. However, please note that it’s possible to hide the Cydia app, so this trick might not always work.
To do a more comprehensive jailbreak check, run a free scan with Certo Mobile Security.