What is interlocking when we refer to a graphical user interface (GUI)?
Basically, it is when only one join can be active at a time in a group of joins.
E.g. you have 4 buttons in a GUI, each that select a source. Only one source can be selected at a time, so you would use interlocking to make sure that only one join can be active at a time. When a different join is activated, it first deactivates all other joins in that group.
These files are designed to help you implement interlocking in your projects.
Below is a screen shot of the .gui file
The instructions are included in the GitHub readme and the Interlock.js file.