Answer:
The central idea is that year after year, black people were assaulted and injured or possibly killed. All that happened was the public accepted that they were inequal to American police and tried their best to avoid trouble with them.
Finally, people are fed up of this and are protesting for their rights. This presents the idea of them quoting "we are sick of it". They are tired of public inequalities to the US police - how police are allowed to just kill a person and get away with it.