Generally, when talking about matrices, I is the identity element. ie. the element such that for any matrix A, A*I = A.

It's the integer equivalent of 1 under multiplication.

ie. if A is an integer A*1 = A.

For 2x2 matrices the identity has

[ 1 0 ]

[ 0 1 ]

for 3x3s it's

[1 0 0 ]

[0 1 0 ]

[0 0 1 ]

And so forth. I don't know how to draw matrices in this thing... sorry...