Complex sentence: While a lot of music in the 60s and 70s voiced left-wing opinions, progg was different in the sense that it took a strong socialist stance that often promoted communism and a radical view of how traditional forces in society were oppresing the proletariat.
Split into simpler sentences: While a lot of music in the 60s and 70s voiced left-wing opinions, progg was different in the sense that it took a strong socialist stance. It's "followers" were often positive to communism and shared radical views of how traditional forces in society were oppresing the proletariat.