I read that sin(2x) and sin(3x) are orthogonal to each other.

In general if I want to check if two functions are orthogonal or not I must integrate their product

First: why the integration of their multiplication (not their addition for example)?

Second: Orthogonal means Perpendicular. But I can't figure out how sin(2x) and sin(3x) are perpendicular

Thank you for your help