The first gift of Christmas is a present Santa Claus gives annually on Christmas Eve. Every year, the Polar Express picks up children from around the world and takes them to the North Pole where Santa chooses whoever he feels is worthy enough to receive the first gift, which can be anything of their choice.
One year, Hero Boy is chosen for of his strong believe in Christmas, which in the film, he was previously unsure about. He decides to have one of the silver bells from Santa's reindeer. These bells are special as they can only be heard by those who have a strong faith and believe in the Christmas spirit. Typically, as children grow older they can no longer hear the bell (as they lose faith and belief), but Hero Boy was one of the few who could hear it all the way throughout their lives.
After Hero Boy receives the bell, he and the other children get back on the train for the return trip. The other children ask to see the bell, but Hero Boy only feels a hole in his pocket when he tries to reach for it. One child suggests they go outside to look for it, but the train was already moving. Fortunately, the next morning, Hero Boy finds the bell in a present with a note from Santa saying it was in his sleigh the whole time and that he should fix that hole in his pocket.