Robot tax is a proposition that employers who replace employees with automated workers be required to pay a tax and the funds allocated to assist the displaced workforce. The purpose of such a tax would be to slow the advance of automation, while helping to mitigate the effect of job loss.
There are a couple of ways a robot tax could work: The business could be required to pay the tax for each robot based on the displaced employee's salary; automated workers that replace humans would be taxed at the same rate as the employee would pay income tax. Alternatively, a business might simply be taxed for using a robot, without any correspondence to employees' salaries. In either case, the tax collected would be used to help displaced workers through initiatives such as retraining and a universal basic income.
One problem with implementing a robot tax is the question of how we define a robot. Within the field of robotics, a robot is a physical system that performs tasks with some degree of autonomy. If an android robot performs the same job during the same hours as a human, the equation is clear. (However, robots don't require the same breaks as humans and can work longer hours and back-to-back shifts.)
Many systems performing tasks that were formerly conducted by humans are what is being called software robots, and there are no clear criteria for differentiating a software robot from many other types of software. Furthermore, that software will typically perform some portion of the work of multiple employees rather than simply performing the same work as an individual employee. Given the efficiencies of software, one such system (or multiple, different systems) could replace a significant number of workers.
Bill Gates explains why he thinks a robot tax is a good idea: