I always thought it was about living people with dead hearts, people who used to have dreams but never really got the chance to fulfill them. "Kids that I once knew" makes it all the more painful if you knew these people personally; you knew what they wanted in life and are sad that they didn't get it. "It's hard to know they're out there, It's hard to know that you still care" could mean that you don't really recognize these people anymore ("once knew") and that they've become pretty apathetic; perhaps they've accepted that this is the most their lives will ever amount to. With their hearts dead, maybe they're just waiting for the rest of their selves to die.
I always thought it was about living people with dead hearts, people who used to have dreams but never really got the chance to fulfill them. "Kids that I once knew" makes it all the more painful if you knew these people personally; you knew what they wanted in life and are sad that they didn't get it. "It's hard to know they're out there, It's hard to know that you still care" could mean that you don't really recognize these people anymore ("once knew") and that they've become pretty apathetic; perhaps they've accepted that this is the most their lives will ever amount to. With their hearts dead, maybe they're just waiting for the rest of their selves to die.