The left square bracket symbol is used in mathematics to group the contents of a matrix together. It is also used in computing to represent the start of an array. For example, the left square bracket is used in the expression below to represent the start of the matrix.
In combination with the right square bracket, the two brackets group the contents of the matrix together.
In math, a matrix is a function that maps between two vector spaces. A matrix can also be thought of as a shorthand way to write a system of linear equations.
An array is a computer programming construct for storing contiguous data. The data can be accessed through indexing the array with the location of each element.