What do you think is meant by the last line of the story? How does it relate to one of the themes in “Games at Twilight”?

The final sentence of "Games at Twilight" is the following line.  

He lay down full length on the damp grass, crushing his face into it, no longer crying, silenced by a terrible sense of his insignificance.

Ravi has just attempted to claim victory in the game, and all of the other children merely cast him aside. They do not believe that he has earned victory, because they have all moved on to different games. In fact, all of the other children completely forgot about Ravi and the fact that he was still playing the game.
A theme of "Games at Twilight" is the interplay between fantasy and reality. When the reader first meets Ravi, it is fairly clear that he is not happy about his location in the hierarchy of children. He's cowering in fear by himself with his finger in his nose and swallowing snot balls. That is not the image of a strong leader type. Once Ravi has found a good hiding place, though, his thoughts wander into the realm of fantasy. He begins to envision what it will be like to win the game. He imagines that all of the other kids will praise his efforts, and he will be crowned a champion of sorts.

What fun if they were all found and caught—he alone left unconquered! He had never known that sensation. . . He hugged his knees together and smiled to himself almost shyly at the thought of so much victory, such laurels. 

Ravi convinces himself that his fantasy will become his reality when he reaches the "den." Unfortunately, the reality of the situation is the complete opposite of his fantasy. Ravi is not praised at all. Instead, Ravi is ridiculed by the other children.

"Stop it, stop it, Ravi. Don’t be a baby. Have you hurt yourself?''

The reality of the situation is that Ravi is just as insignificant to the other children as he always was, and that is what the final line of the story is pointing out to readers.