Complex sentence: The prefix "zetta" indicates the seventh power of 1000 and means 10 21 in the International System of Units (SI), and therefore one zettabyte is one sextillion (one long scale trilliard) bytes.
Split into simpler sentences: The prefix "zetta" indicates multiplication by the seventh power of 1000 or 10 21 in the International System of Units (SI). Therefore one zettabyte is one sextillion (one long scale trilliard) bytes.