Spectrograms is a method of hiding images inside of an audio or video file. Basically the y-scale is represented by frequencies, while the x-scale represents time. The result is weird sounds and strange sound patterns. For example the viral 11B-X-1371 video used this method to embed creepy images. To view spectrograms, you can use a tool such as Audacity and there switch the visualisation from Waveform to Spectrogram. If you are analysing a video file, you need to extract the audio before opening it in Audacity.