Please read this explanation carefully:
Anachronisms are things that are placed in the wrong time period.
They crop up in pictures, or in writing, and are often the result of not researching a topic well.
For example: a writer could refer to William the Conqueror phoning London. As the telephone was not invented until 1867, this would be an anachronism.
Or, a painter might paint a picture showing people cooking their dinner on an electric cooker in 1450. As electricity was not invented until the late nineteenth century, this would be an anachronism.
Sometimes anachronisms happen by mistake.
For example: a film director making a film about the Victorians might, by accident, film an aeroplane in the sky. As aeroplanes were not invented until the twentieth century, this would be an anachronism.
Or, the sound recordist might, by accident, record the sound of a mobile phone ringing. As mobile phones were not invented until the late twentieth century, this would be an anachronism.