Complex sentence: Spirit of'69 is a phrase used by traditional skinheads to remind other skinheads of the subculture's heyday in 1969, when ska and reggae music was at its most popular with skinheads, and when there was a heavy emphasis on mod-influenced style.
Split into simpler sentences: Spirit of'69 is a phrase used by traditional skinheads to commemmorate the skinhead subculture's heyday in 1969. The phrase celebrates the late 1960s, when ska and reggae music were the most popular with skinheads--and when there was a heavy emphasis on mod-influenced style.