If both i and j are scalars, then A[i, j] refers tot he element of A at -th row and -th column.
Such an indexing expression will return a square block of the indexed matrix. For example, A[3:5, 7:10] returns a matrix whose elements are all the elements of A that lie on row 3, 4, 5 and column 7, 8, 9, and 10, namely