What does 128PB mean?

A PB stands for Petabyte. It is the next logical size increase from Terabyte. Depending on if you are referring to manufacturer's storage or computer programmer's definitions of storage, a PB is:

Manufacturer: 1PB = 1,000TB (Terabyte) = 1,000,000GB (Gigabyte) = 1,000,000,000MB (Megabyte) = 1,000,000,000,000KB (Kilobyte) = 1,000,000,000,000,000 Bytes.

Programmer: 1PB = 1,024TB = 1,048,576GB = 1,073,741,824MB = 1,099,511,627,776KB = 1,125,899,906,842,624 byes

To arrive at 128PB, simply take a unit of measurement above and multiply that number by 128.