[]               Index Operator                                 MASM Operator

expression1[expression2]

Adds the value of expression1 to the value of expression2.  This
operator is the same as the plus-operator ('+'), except expression1 is
optional for this operator.

Notes:     If expression1 is specified, it can be an integer value,
relocatable operand, or absolute symbol.  Expression2
can be an integer value, relocatable operand, or
absolute symbol, unless expression1 is specified, in
which case expression2 can not be a relocatable operand.

This operator is generally used to access individual
items in an array.  For example, the following will
access the third byte of a character string.

MOV       AL, STRING [2]

See also: .
See also: +

[] Index Operator