From phpBB Development Wiki
So what are ledges
Ledges are hook locations. So what are hook locations? First you need to know what hooks are.
Ledges are the code injection points in the code. To use a hook the code inside the hook needs to be injected into the code in the right place. This can go in anywhere in the phpBB code base but it needs to have a ledge there.
How to add a ledge
You would add a ledge as so:
Lets dissect that code
In phpBB this is automatically loaded, and there is also code that will load all the hook files (hook/*.php). It registers the hook containers of those files with the hooks controller.
This explains that you are adding a ledge.
This is where you enter the name for the hook. If its in the core it should be prefixed by "core." but if its in an extension you should prefix it with something relating to your extension. This is to prevent clashes with other software.
This is where the input parameters go. If there is one it should be listed as it is in the example, if it is more than one then it should be in an array.
Based on the blog article here