Moire pattern usually occurs when the subject of your photo has repetitive pattern details, which can be lines or dots that exceed the resolution capability of the camera sensor so that the resulting photo shows additional patterns that look strange.
Source : http://belfot.com/
It is usually not visible to the naked eye because the quality of our eyes is of course much better than a camera. But not with cameras, moire patterns are sometimes still often found, especially on subjects that have repetitive, dense and complex patterns such as hair and escalators. Moire usually occurs due to the influence of light entering the sensor and the way the sensor interprets that light.