When one takes a photograph of an object such as a leaf it is a result of light hitting the leaf and then ‘bouncing’ off into the lens of the camera. In the days of film the film would be developed and the negative would be placed into an enlarger. This would allow light to pass through the negative and enable an image to be enlarged on to the photographic paper.
If one were to place an object, such as a leaf, into the enlarger instead then the image would be very different from the photograph of the same leaf. To start with the colours will be direct opposites of each other as the photograph reverses the colour on the negative but then reverts it on the paper. The photogram misses the first stage and so the image is only reversed once. The other main difference is that instead of light bouncing off the object, as it does with the photograph, the light from the enlarger passes through the object.
The results can be interesting, beautiful and sometimes frightening.