You can find the definition of the mount function on the Linux man page. It has 5 parameters. In this wiki and the Cleaner app, you only need to care about the first two, the source and the target.

To illustrate why mounting can achieve the effect of storage redirect, let's look at an example first. Let's say we have just connected a USB device, and the system has assigned the block device path /dev/sdb1 to it. When we enter the command mount /dev/sdb1 /mnt/usb, we can access the USB device through /mnt/usb. In this example, /dev/sdb1 is the source, and /mnt/usb is the target. When we access /mnt/usb after mounting, it will jump to the file system of the USB device, so we can view the files in the USB device.

What's the relationship between this example and storage redirect? In fact, it is mentioned on the Linux man page that the source can be not only a block device but also a file or a directory. If we replace the USB device in the above example with a directory in the Android storage, and replace the object access the directory with an application in Android—yes, that's the rationale of storage redirect.