A tools programming position is generally about being a force-multiplier for the rest of the development team. The exact nature of the work will of course vary widely from studio-to-studio, depending on the individualized needs of that company.
Primarily, however, you would be tasked with creating software that was going to be consumed by other developers within the studio, and the role of that software might be to:
- author assets or content (levels, models, items, spells, whatever) for the game directly
- act as a bridge between your studio's game or build pipeline and another content creation tools (for example, exporters for Maya).
- be part of automated build processes that transform the source game assets into their final compiled forms for distribution with the retail game
- assist developers in day-to-day boilerplate tasks, such as syncing to particular builds of the game or transitioning between branches
It's also important to note that in some studios this is considered an extremely entry-level position, because it ends up involving mostly thoughtless grunt work (moving buttons around on a UI for a designer, perhaps). In others, it's exactly the opposite, because it involves the care and feeding of mission-critical build and deployment pipelines for live shipping MMOs.
The range of responsibility and expectation is large (although really, this is true of almost any position in the industry), but the overall goal is usually the same: keep on the lookout for any bottlenecks in the organization, and then buy, build or retrain until you have optimized that bottleneck.