A memoryview is essentially a generalized NumPy array structure in Python itself (without the math). It allows you to share memory between data-structures (things like PIL images, SQLlite data-bases, NumPy arrays, etc.) without first copying. This is very important for large data sets.
With it you can do things like memory-map to a very large file, slice a piece of that file and do calculations on that piece (easiest if you are using NumPy).