It has been for quite a long while that mount bind is used on rooted Android devices to tackle internal storage issues. They are used for binding of two different folders together, making two different paths completely identical. Any operations that are done on the internal storage path will take affect on the external storage path instead (and vice-versa).

This tool is aimed to easily allow any users to easily generate an init.d script that allows them to bind two different paths during boot.

Different compared to the Linux PC counterpart, Android's "mount" binary has a different syntax. Here is the man page for mount on Linux PC (or run man mount on your terminal instead for better and up-to-date information). Example of the difference are as follows

# PC syntax
mount --bind source target
# Android syntax
mount -o bind source target

An app that exactly does this is Foldermount. Check the app out. If you are wondering what is the difference, the app mounts the folder after boot completion, while an init.d script runs during boot-up.

This tool is also a web version of an upcoming Android app counterpart.