I guess you mean something like a testing harness...?
That's a compilation of functions or scripts (covering more or less basic functionalities of your AUT) that will be used by your "real" testing scripts.
Example: If you have 10 testcases that require to open a special window in your application, you write a function that opens that window. All 10 testcases call this function to open the window.
Advantage: If the way of opening this window changes, you'll only have to change it once.