Complex sentence: Since an emphasis during the Summer of Love was enlightenment and personal development, LSD--also known as acid--was popular, since the perception-altering effects of acid were often seen to be a path to self-discovery.
Split into simpler sentences: Since an emphasis during the Summer of Love was enlightenment and personal development, LSD (also known as acid) was popular. The psychedelic and perception altering effects of acid were often interpreted to be a path for self-discovery.