Because I can tell you what it isn't: Hope is not some ephemeral feeling that things will get better, someday. Hope is not blind faith that a savior will one day take you to a paradise. Hope is not vain trust in some undefined cosmic force that may one day save you. Hope is saying that if the world won't get better, then damnit, you'll make it better. Hope is seeing the world for what it is, warts and all, and the good people in it making it better in whatever small ways they can. Hope is knowing that even if nothing seems to change, you are still turning the wheel of progress, and no matter how slow it turns it does nonetheless.
I know that this likely sounds like a bunch of recycled platitudes, but I hate seeing people say that nothing will get better, or that the current situation is hopeless, or that nothing can be done. It's likely a symptom of my larger loathing for the concept of 'unsolvable problems', but I just can't stand seeing people give up! Something can always be done, no matter how bad the situation is, no matter how bleak the future, no matter how abysmal the circumstances. I'm not saying that you have to feel unwavering optimism, gods no, but please, don't just lie down and rot. I understand feeling pessimistic, I understand being depressed, I understand wanting to give up, but I can't understand dismissing the possibility that anyhting could get better. Stars, doesn't being endlessly negative get tiring? Are you not exhausted? Trying to understand why people refuse hope is like trying to comprehend Cthulhu to me, some fundamental part of my mind just rejects the idea entirely.
Ultimately, what I want to convey is this: In the long run, I'm pleased to say, the optimists have it! The world can and has improved, heroes can and have existed, and while it is entirely reasonable to expect bad things to happen or to feel anxious or depressed about the future, you can do something to make it better; don't forget that.