This period begins after the first 3 weeks of age, when it begins to hear and see. Before this time, these senses are not quite developed. From this point up to 12 weeks old, it develops the sense of fear, which is fundamental for its future socializing development and on the contrary, if they don't know HOW to act, to future phobias.
When puppies live with their siblings and mother, they get used to other dogs. As a result, it will have better relationships with other dogs.
This puppy is socializing with its mother. This is essential for its future balance and behaviour.
During this period, we need to get them used to us as well. We need to know that an adult and a child are completely different to them. They need to become used to the crying of a baby as something common and normal.
This is a period for recognizing different species.
Those animals which are isolated during this period will probably develop behaviour disorders.