Imagine a programming language that gives you three different ways to add 2 numbers:

1 + 2;

std::add(1, 2);

std::int(1).add(std::int(2));

That's is not orthogonal at all, since we have 3 different ways to do the same thing for no discernible reason.

An orthogonal interface is one where there are fewer reasonable ways to do something, and each function/operation has it's role clearly defined, it doesn't try to do things unrelated to its main role, and the interface doesn't mix different layers of abstraction (like intertwining OOP and arithmetic in my 3rd example, when there already are native arithmetic operations).