"dim a(4,4)" would give you a 25 block peice of data with 5 rows and 5 columns. (counting 0 of course)
So you would have
0 1 2 3 4
0 x x x x x
1 x x x x x
2 x x x x x
3 x x x x x
4 x x x x x
So if you said "a(2,4)=69" this would happen:
0 1 2 3 4
0 x x x x x
1 x x x x x
2 x x x x x
3 x x x x x
4 x x 69 x x
And if you said "FOR X=0 TO 4:FOR Y=0 TO 4:a(X,Y)=X*Y:NEXT Y:NEXT X" then you would get this:
0 1 2 3 4
0 0 0 0 0 0
1 0 1 2 3 4
2 0 2 4 6 8
3 0 3 6 9 12
4 0 4 8 12 16
You can also use more than 2 dimensions like "dim a(4,4,4)" which would give you a 125 block peice of data. (5x5x5)