A social movement is a large, organized group action (ex. walking or protesting) for a cause, typically political.
A currently occurring example would be March For Our Lives, a social movement led by the United State's youth to revise and reform the 2nd amendment.
A famous past example of a social movement would be the Civil Rights Movement in the 1950s and 60s led by Martin Luther King, Jr. to secure legal rights for African Americans in the US.
Hope I could help!
