After being pressed and pressured by her mother with the topic of the gentlemen callers, Laura finally confided in her mother that she had always liked someone whom she deems to be some form of crush she has had since adolescence.
At this time, Laura takes out the high school yearbook and she points at the picture of Jim, who is a popular and hip class president. We know that he is important to her in that a) she had read that he was engaged to another woman and thought with sadness that he was married, b) she remembered his voice to this day, and c) she remembers how he used to call her "Blue Roses". All this was huge to Laura, who was an extremely shy girl.
Surely Amanda would have wanted to have a man like that ask her daughter out, but Laura considered herself unworthy for being "a cripple" which her mother did not allow her to admit to be. In the end, Jim did visit the house as per Tom's request, but was engaged to marry someone else. This were sad and sour news for Amanda who had a very unrealistic hope that Jim would turn out being Laura's so much sought-after gentleman caller.
We know that the boy in the yearbook was important because the way Laura talked about him. She carried so much passion, also, she knew the pages where he was. Also she knew that he was in the play, and his other achievements.