Su Shi's appreciation attitude towards ancient art and culture is described as "ruling six dynasties and reading three generations of books" in "Red Cliff Fu". He divided the historical period into six dynasties and three generations. The Six Dynasties refer to the cultures of other periods, and the Three Dynasties refer to the cultures of the Han, Wei, Jin, Southern and Northern Dynasties, Sui and Tang Dynasties. In Su Shi's eyes, the calligraphy art in these three periods is relatively the best.
To sum up, this sentence means Su Shi's appreciation and love of calligraphy art in the Han, Wei, Jin, Southern and Northern Dynasties, Sui and Tang Dynasties.