Holden's younger brother Allie is a special figure in this novel. Holden has two experiences with death in the book, the most important one being the death of Allie.
When Allie dies, Holden becomes distraut and depressed and we might argue, given evidence from the text, that Holden still has not recovered from the loss of his revered and beloved brother.
Allie was not just Holden's brother. Allie was the exemplar of certain qualities that Holden cherishes and esteems. Allie's baseball mitt relates directly to these qualities.
Like the oldest brother in the family, Allie is interested in writing and literature. He covers his baseball mitt in poetry so that he has something to keep him entertained in the outfield.
Allie's love of poetry is evidence of his integrity, which, for Holden, is an attribute of Allie's ability to believe that there is beauty in the world.
This is an aspect of innocence. It is innocence that Holden admires most in the world and he finds it only in Allie and his sister, Phoebe. These are the two figures that support Holden, in different ways, as he goes through the breakdown depicted in the novel.
Phoebe is the only human being with whom Holden can communicate except for the memory of Allie, for whom he continually grieves.
The baseball mitt, then, serves to connect Holden to Allie, symbolize the qualities of Allie's goodness (his belief in beauty), and acts generally as a symbol for Allie himself.