It's just an obscure way of accessing array elements, I've never seen anyone use it in real code. A trick I use is to read it backwards, so this is the same as a[1][2] which gives you the 7th element in the array, '21'.
Record 1[a] and a[1] are equivalent and in turn are equivalent to as *(a + 1). The type of the expression is int [4] . The value of the expression is the second "row" of array a. Let assume that thsi row has name b. Then 2[b] is equivalent to *( b + 2 ) and will give the third alement in this second row that is 21.