Parallel play usually begins after 1 year of age. It's a form of activity in which children play next to each other. One child may show interest in what the other child is doing, but they don't try to impact the other person's game and they do not play at the same game togther.
In the autism spectrum, children seem to exhibit more parallel than cooperative play in peer situations. While a group of children may be playing cars, the ASD child may be playing next to the group (perhaps with a car or perhaps with a different toy).
In adults, we see less parallel play and more interactions where people in the group are exchanging ideas, information, jokes, etc. But of course, some parallel activities continue such as one person playing at a crossword while another reads a book. The couple may say they "had morning coffee together" although there was little exchange.
Within the autism spectrum, we may see more parallel play than we do in friendships outside the spectrum. These activities occur "together" but without personal exchange. There may be comments about the task ("How is the book?"), but there would little exchange of thoughts, ideas, opinions, or emotions.
After activities with others, the ASD individual may still know very little about the other person. To him/her, the togetherness may have been about a task or activity, more than any exchange of minds. Parallel play in adulthood continues and may be more common for the autistic adult.