Maple uses I to represent one of the square roots of -1, with -I representing the other, for computations over the complex numbers.

Arithmetic expressions involving I and other numeric constants are automatically evaluated.

The evalc function can be used to symbolically manipulate complex-valued expressions.

The evalf function can be used to numerically evaluate complex-valued expressions.

The evalhf function can be used to numerically evaluate complex-valued expressions using the floating-point hardware of the underlying system.

I is implemented as Complex(1), and therefore, unlike many other Maple constants, type(I, name) returns false.

Since the literal expressions "sqrt(-1)" or "(-1)^(1/2)" do not appear in the representation of I, or any complex number in Maple, type(I, radical) returns false.

If you want to see this complex constant displayed as another letter (for example j), use interface(imaginaryunit=j).  See interface for  more information.