I've been reading around online and through books and still can't quite figure out what exactly tool pipelines are within the game industry.
The tool pipeline is the set of tools that an asset has to go through from the original creation to the moment it's used in the game.
At the same time
All of these tools have a purpose, and must be executed in the proper order on the assets.
Each company has its own unique set of pipelines for different purposes (e.g. one for 3d models, one for the audio, one for the AI, etc.): they evolve out of the needs of the company and the engineers who think about them.