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.
$A=[ad be cf ]$
In combination with the right square bracket, the two brackets group the contents of the matrix together.
Symbol  Format  Data 

[  Unicode 

$[$  TeX 

SVG 