1. The 'Customize Analysis' screen allows you to configure which items CCleaner will scan for. The 'APK Files' item is for scanning of APK files (these are Android Installation packages). In many cases these are left on the device after installation if they have been downloaded from a third party or copied onto the device.
2. Locking an item prevents CCleaner from removing it. Based on your screenshot, if you lock an app in the processes screen then CCleaner will not kill that process. Under any of the file sections (Downloaded, APK) locking a file will mean that CCleaner will not remove it as part of the clean process.