This one is about as straightforward as it gets. Scrooge needs to learn to appreciate Christmas as a holiday centered on kindness and generosity towards others.
Who shows him why he has becomes the man he is today? Ghost of Christmas Past. Who teaches him what the Christmas season should actually look like? Ghost of Christmas Present. Who shows him what will happen if he doesn't get his act together? Ghost of Christmas Yet to Come.
They are as literal a series of guides as Virgil is to Dante in the Inferno.