The Robin Hood Tax or Financial Transaction Tax (often labelled an FTT in the acronym soup of financial jargon) is a tiny tax on the financial sector, typically only a fraction of one percent. The key reasons for its implementation we can see from the original folk tale,
“consider the entire Robin Hood myth, the ideal remains familiar in our outlaw-free world. The myth requires a great mass of heavily taxed poor people who work terribly hard for little reward. The profits of their labour, and the taxes they pay, go to support a small number of lazy, arrogant rich people who live in big houses, wallow in luxury, and have no need to work. Any attempt to resist, let alone change, this unjust system is crushed by the weight of a vast private-public bureaucracy, encompassing the police, the courts, the prison system, the civil service, large property-owners and banks, all embodied in the ruthless figure of a bureaucrat-aristocrat, personification of the careerist-capitalist elite, the sheriff of Nottingham.”
So, the idea of an FTT is to take from the wealth of the financial markets in order to provide societal goods, health and education or social housing for example.
A more complete explanation of the Robin Hood Tax can be found in the documentary.