Simplify the following text for a Grade 10 native English-speaking person.
Original: Students for a Democratic Society ( SDS ) was a student activist movement in the United States that was one of the main iconic representations of the country 's New Left . The organization developed and expanded rapidly in the mid-1960s before dissolving at its last convention in 1969 .
Simplified: The Students for a Democratic Society ( SDS ) was a student activist movement in the United States in the 1960s ( 1960 to 1969 ) .