The titles of these books give a clue to what the main storyline is about, as all titles usually do. The message of each of these stories is also reflected in the titles. For example, "The Rocking Horse Winner" reflects the message that a child, a small child, on a rocking horse, was able to win something. But it is a bit absurd; how can a rocking horse win anything? How can a child's toy be any part of a serious adult world of money and winning? And, that is one of the messages of the story; a child takes on adult responsibilities and stresses, and "wins" his goal, but in the end, the pressure of trying to be an adult takes his life. So, it is a "winner", but done with a rocking-horse, or a child. And so, we see the absurdity of that entire situation, and how impossible it really is.
For "The Lottery", the title conveys the message of the story that the person chosen to die is chosen in a completely random and impersonal way. When one thinks of a lottery, they think of large odds stacked against their favor, but if they win, it's good luck. In this story though, it is bad luck to win the lottery; it is a reversal of expectations. The lottery is in fact the most important "character" of the story, because a person's life lies in its hands. So, the lottery is not only the title, but the key character, key determinant of human life, and the entire social system around which this society is based. A lottery is impersonal, just as their selection of person is impersonal.
I hope that those thoughts help a bit; I provided links below to further discussion of theme and messages for each story, and those should help too. Good luck!