NumPy (short for Numerical Python and pronouced num-pea or num-pie) is a third-party Python module for numerical programming. Any time you have collections of numbers, either one-, two- or more dimensional, then NumPy is likely to help you out. It provides you with a large suite of tools, algorithms and techniques. It is one of the most commonly used Python packages around and is used in the majority of Python-based scientific software.
It complements the other major tool in this area, Pandas, which is a data analysis package. To learn more about Pandas, have a look at the Introduction to Data Analysis in Python course. NumPy is well-suited to dealing with n-dimensional grids of numbers, while Pandas is best for tables of data. They work well together, and in fact, Pandas is built on top of NumPy.