Cracked apps are modified applications that work around the licenses process (stolen in other words), just like pirate software on your computer. You get not support or update and of course you are not supporting the developer of the app. If you have cracked apps showing on Cydia is because you added a source that support this type of software, keep in mind that "anything" can be bundel with this cracked app (including trojans and rootkits to steal your information, or open remote connections)