A one dimensional (1d) array is like a list of items:
A two dimensional (2d) array is like a rectangular matrix of items:
You get can an element of a 1d array with one subscript: list is the int "12" (we count from 0 in Java). But you need two subscripts to get an element of a 2d array: matrix is the int "9". 2 is the row number, and 1 is the column number. Make sense?