What technique can I use to remove useless information from an image? Is there any way to remove useless information (unimportant details) from an image?

If, for example, we have a specific image, so this contains both useful and useless information. If this image can be recognized directly without the useless information, the unnecessary information can be removed (e.g., illumination, some noises, variations, etc.).
Although you did not spent even a minute in researching the question I will post an answer to it. There are multiple ways; I will try to demonstrate it using wavelets in Mathematica.
So, first of all we need an image.

img = ExampleData[{"TestImage", "Mandrill"}]



  


Then we apply the DiscreteWaveletTransform using the HaarWavelet

dwd = DiscreteWaveletTransform[img]


We plot the transformation

WaveletImagePlot[dwd]



  


We threshold it, i.e. removing the noise according to some criteria

wtd = WaveletThreshold[dwd]


Again plot the transformation and the filtering

WaveletImagePlot[wtd]



  


We now inverse the transformation

iwd = InverseWaveletTransform[wtd]



  


And visualize the differences between the two images

ImageDifference[iwd, img]