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 like so:
Let's dissect that code
This is the phpbb_dispatcher object, which is automatically loaded with the page. This object registers the hook containers of found within hook files (in the hooks/*.php directory) with the hooks controller.
This method adds 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 a unique string related to your extension to prevent clashes with other extensions.
This is where the input parameters go. This will either be a single variable or an array of variables, depending on the ledge onto which you are hooking.
Based on the blog article Explaining Hooks