The baseball glove of Allie is mentioned at the end of Chapter Five, and is the topic that Holden chooses to write the descriptive composition on for Stradlater. Note the description that Holden gives us of the baseball glove of his dead brother:
My brother Allie had this left-handed fielder's mitt. He was left-handed. The thing that was descriptive about it, though, was that he had poems written all over the fingers and the pocket and everywhere. In green ink. He wrote them on it so that he'd have something to read when he was in the field and nobody was up at bat.
However, far more important is the way that the baseball glove acts as a powerful memory of Holden's dead brother and the love that he has for him and the way that it demonstrates Holden's attachment, but also his problems at trying to accept his brother's death. Note the way in which he tells us--almost as an afterthought--that the night his brother died, he broke all the windows of the garage with his fist, "just for the hell of it":
It was a very stupid thing to do, I'll admit, but I hardly didn't even know I was doing it, and you didn't know Allie.
The fact that he was going to be psychoanalysed for this behaviour shows how violent and destructive it was, and also points out that Holden still has some deep unresolved issues regarding his brother's death and his acceptance of it. The value of the glove therefore lies in what it tells us about Holden and his psychological state: he is clearly a very angry young man, and we can see this in the act of self-harm he committed after his brother's death just as we can see it in the way he rails against all the "phonies" he comes across.
Allie's glove symbolizes love and empathy in the novel. Holden has trouble sympathizing with others and fitting in. Allie was the one person Holden could truly identify with and care for. When Allie died, Holden punched out the windows until his hand turned bloody. He can't even make a proper first because of that. Holden is still very much grieving for Allie and carries Allie's glove around as a sentimental reminder. Allie's death profoundly shaped Holden and that is why the glove is an important motif in the novel.