The capital Latin letter I is used in math to represent the identity matrix sometimes with subtext to represent the size of the matrix.
For example, the symbol appears in an expression like this:
In plain language this means that the expression is equal to the square identity matrix with a width and height of two. More generically the expression represents the identity matrix with a width and height of .
The Latin letter i is used in mathematics in the definition of a complex to represent the square root of negative one.