Kevin is left at home…alone. Bam. There's your title explanation.
But, to go into a little more depth…
Being "home alone" is both a childhood fantasy and a childhood fear: you're free from your parents' rules, but you're also without their protection. There's something both exciting and scary about the idea. Most little kids would probably say that'd it would be fun to be home alone—but if it actually happened? Cue the waterworks.
For Kevin, that's how the experience plays out. He reacts first by celebrating his newfound solitude—eating a gigantic bowl of ice cream, tobogganing down the stairs, and watching violent movies—but then he starts to freak.
He's frightened when the burglars try to get into the house the first time, though he manages to scare them off by turning on the basement light. And he's irrationally afraid of old Mr. Marley, the snow shovel guy. He doesn't have any parents to guide him through these fears. He has to become self-reliant.
Even though leaving a kid "home alone" is, from a parent's perspective, a bad thing, it actually makes Kevin a fuller person than he previously was. At the beginning, he was just a whiny little ball of incompetence—if still likeable, for all that. But as the pressures of living alone affect him, he learns to access his inner hero. He shops for himself, does laundry, and takes care of his personal hygiene. This gives him the confidence he needs to take on the burglars at the end—creating a maze of booby traps that might've been beyond the conception of the average adult.
So, while being home alone is a crisis and a conflict—and conflict is the essence of plot—it also gives Kevin the tools he needs to navigate his world after that conflict has ended.
And you thought this would be a one-line answer, huh?