All animals, including dogs, have a special sensitive period at the start of their lives. During this time they learn to accept things around them so that they are not afraid of them later in life. In puppies, this window of opportunity closes at about 12-16 weeks of age.
Puppy socialisation involves meeting and having pleasant encounters with many different adults, children and dogs. During this process they will also get used to a wide range of events, environments and situations.