In 1955, Rosa Parks ignited the beginning of a revolution by sitting in the white section of a bus, thus the beginning of the Montgomery Bus Boycott. After an exhausting day of work, she got on the bus going home and sat in the first row of the "colored section", which was given to whites if the bus began filling up. She was ordered by the bus driver to move to the back of the bus but refused, resulting in her getting arrested and fined $10. This was the spark that ignited the following chain of events in the US leading up to the Mississippi Freedom Summer in 1964.
Most African Americans lived in incredible poverty, were indebted to whit banks, and were kept in check by both the KKK (and any groups similar) and officers. The one's who'd challenge these conditions were either killed, beaten, raped, tortured, fired from their jobs, or kicked out of their homes. SNCC and CORE leaders thought that bringing well-connected white volunteers would attract media and make the government enforce the civil rights laws that local officials usually ignored. They also helped the African Americans organize the "Freedom Election".
In 1964, less than seven percent of black mississippian's were registered to vote, compared to the 50-70 percent in most southern states. Segregationist white establishments did anything and everything in their power to keep African Americans from voting even though in most rural counties, they made up majority of the population. For years, local civil rights workers relentlessly tried to get more African Americans to vote, failing each time to do so. The ones who wanted to vote had to go through local registrars, white functionaries that would put their names in the papers and let their employers and banks know. Even when threatening them or putting their jobs on the line didn't work, the policies would still keep them off the rolls.
Black Mississipian's were banned from Democratic Party election meetings, so they challenged the party's all-white delegation to represent the DNC that August. Since they weren't able to vote in the Party's, this meant they were banned from participating in anything political. Thus the rise of the "Freedom Election" that was held that November, challenging the all-white representatives once again. Sixty thousand black Mississippian's risked their lives by voting in the "Freedom Election" that was held at the same time as the regular national elections.
The day after June 15, 1964, two white male students from New York and a local black male disappeared. Their bodies weren't found for six weeks and people became certain that they were murdered. People of both races, mostly African Americans, were getting constant threats that would lead to violence at times. Eventually everyone grew annoyed and impatient with how slow the investigation was going and distrust had grown greatly between white and black workers.