A phrase used usually in a mocking, aggravated and ironic sense to lend light to some rampant lossage in the operation of some Government entity, program or office. The Big Idea is that your tax monies are being used (read: mis-managed) to orchestrate these, yet your are seeing exactly zero benefit from it.
The phrase comes from, in my experience, roadside signs around sites of road work and contruction. Big signs that say "YOUR TAX DOLLARS AT WORK!", followed by seeing the actual work site either fallow or full of workmen who are, in fact, not actually doing any sort of work (Men at work = Dudes Partying).
A beautiful example of usage that, in fact, inspired me to write this just took place in #e:
* moxiee just watched a special on court tv where the girlfriend of a dealer got more time than he did, just for answering the phone
<xunker> moxiee: Your tax dollars at work.