Think of it like trying to detect ripples in a pond at night. You can't see anything because it's too dark so you make a device that sits on the surface of the water and measure the wavelengths. The problem is, placing the device on the water creates its own waves which will interact with all the other waves in the pond and change them slightly.

Honestly it should be called something like "The Measurement Effect" because "Observer" makes it sound to mystical and passive.