Collective Activism provides a percentage bonus to your overall Culture output for each city-state of which you're Suzerain. Despite being toned down in Gathering Storm, it's still immensely helpful if you're on friendly terms with several city-states and are going for a Cultural Victory.
Civilopedia entry Edit
The problem with collective activism, according to the American economist Olson in 1965 AD, is that coercion or some incentive must be present in order to get a group of individuals to act in the common interest. In the latter half of the 20th Century, a number of activist leaders (Gandhi, King, Mandela, etc.) called for non-violent collective actions – boycotts, rallies, strikes, sit-ins, marches, petitions, educational programs, and such – to effect social, economic, political, and environmental changes. In industrial nations with semi-democratic governments, these have proven effective (sometimes). Collective activism has now gone viral, with social media and the internet now encouraging public support (financial, if nothing else) for these movements.