They actually have various meanings, but mostly they represent past, present and future. The Ghost of Christmas past represents all the memories of the past, that's why it's depicted with a glowing head to emphasis the mind. The Ghost of Christmas Present represents all the values that are cherished during the present time and those that Scrooge has been lacking such as - Christmas spirit that includes generosity and empathy. Lastly the Ghost of Christmas Yet to Come does sort of symbolize death. It is actually more the fear of death and the price he would pay if he didn't fix his moral issues, sort of like a moral reckoning.
So together they all represent a journey of introspect, where Scrooge analyzes his past where he sees why he turned out to be the way he is, then sees the present and analyzes how his decisions affect others and himself in a negative way and then thinks of what his future might be if he continues down this path.