Hello Adrian,
That is a reference to a velocity macro (velocimacro). This is something that would be defined somewhere in your velocity code. They are global and accessible anywhere so locating their definition can be a challenge.
One potential way to find the definition would be to use the Search portlet to search VTL files for that reference.
Go to the Search portlet and enter these filters
Type - File
File Name - *.vtl
Metadata - content:navToolMenuLevelMolli
The first 5000 characters of any text based file are indexed in dotCMS under the metadata field 'content', so you can search files for content using 'content:{WhatYouAreLookingFor'. Of course if the macro is defined after the first 5000 characters of the file this will not find it.
Macros are usually defined in files that are loaded consistently to ensure they are present. This might be located in your theme files like header.vtl. If your search doesn't turn up results I would begin looking in those files. You may find that they parse OTHER files that are macro files, so you will then need to look in those files as well.
Beyond that, ask around your organization. These were created by someone, and that person can shed more light on it.
Let me know if this helps!