A matrix is a mathematical object consisting of a rectangular array of (in blender's case) numbers. Matrices are used in 3D graphics for computing coordinate transformations. Blender uses matrices in the material node system for this purpose and internally for similar purposes. The internal representation can be accessed through bpy. Use this tag for questions about matrices in either the node editor or python.
Wikipedia define matrix as
a matrix (plural matrices) is a rectangular array or table of numbers, symbols, or expressions, arranged in rows and columns, which is used to represent a mathematical object or a property of such an object.
Blender uses numeric matrices for computing coordinate transforms and exposes that use in the material node editor and in the Python
Use this tag for questions about matrices in either the node editor or python.