That very much depends on the company. 'Tools' is a pretty broad description. Sometimes it means writing custom Maya/Max plug-ins to set up custom shaders, level editing, data export/import etc. Other times it involves writing custom MySQL asset servers with some form of C#/HTML5 front end. It can also mean writing large scale stand alone level/character authoring tools.
Generally (imho as a former tools team lead) the best people know about Maya and/or Max, both in terms of their API's, and as a user (because understanding how the artists work helps you write better tools). They are also very good at writing solid, stable, well tested C++/C#/Python code. Extremely good knowledge of Matrices/Quats/Vectors is an absolute must (because attempting to flip coordinate systems, whilst baking out scale & pivots, is one of those annoying and common tasks). Experience packaging assets for use in runtime is really useful (fix/unfix pointers, 16byte aligned data structures, data compression, etc). Knowing how to use VTune always comes in handy (because everyone hates long asset processing times).