If you’re watching or reading a story that has one central character or hero acting in a single plot while surrounded by a group of minor characters, you’re encountering a narrative that has a single strand. Even though there is an antagonist who probes and tries to get in the way of the protagonist, a single strand narrative will always keep the focus on the protagonist and highlight his story and adventures above anything else going on. The quintessential superhero, Superman, is a fine example of a story that’s dominated by a single narrative strand.
Many works are made up of multiple narrative strands. Instead of a single hero and a group of supporting characters, a narrative with multiple strands can have two or more isolated groups of characters existing at once. For example, the comic book series X-Men does not have one central hero, but many. By juxtaposing strands -- groups with their own central characters -- the audience will not only be concerned about the action and outcome of each strand, but of how the strands relate to one another.
Many films, short stories, novels, television shows and other art forms juxtapose narrative strands while having all strands interact with one another. In the case of a narrative with multiple strands it’s quite possible that supporting characters in one strand will become central characters in another. Each narrative strand gives the viewer or reader a unique look into a particular group dynamic and story line, and by having multiple stands with a number of central and supporting characters, the interaction of characters in separate strands can make for entertaining and surprising storytelling.
In the case of a story that has three or four narrative strands, it’s possible for all strands to coexist in the same time and space while still maintaining independent central and supporting characters in and of themselves. For example, in the film "Four Rooms," four separate stories are going on -- four separate strands -- but all of them take place in the same hotel. The location and time are what unite the narrative strands.