If something is totally alone, or consists of only one part, it's single, like a single sock in your clean laundry basket that's mysteriously lost its mate.
Maybe there's a single house on a street, or a single red rose in a bouquet. People can be single too, if they aren't attached romantically to another person: "She had a terrible boyfriend, but now she's single." Single can also mean "distinct," or separate from everything else, like when you read every single word of a book to prepare for a test at school. And if you "single someone out," you place the focus on that one person.