This Blog entry is from the Data Structures section in Learn R.
As a matrix is made up of vectors, it is logical to expect it to bear some resemblance in the way selection from a matrix takes place. All subscripting in a separate dimension when specified inside the  square brackets, as separate arguments. The first argument inside the square brackets relates to the row, the next the column.
To obtain the value in a given position of a matrix, in this case two down, three across, type:
Run the line of script to console:
It can be seen that the value 2 has been returned which corresponds to the position specified: