Close to the end of the book, Katniss worries that despite everything the rebels have gone through, it's all been for nothing:
All those people I loved, dead, and we are discussing the next Hunger Games in an attempt to avoid wasting life. Nothing has changed. Nothing will ever change now. (26.62)
We hit the last chapter feeling pretty gloomy. Thankfully, though, the Epilogue seems to counter all these dark thoughts. It suggests that there has been some change in Katniss's world.
In the Epilogue, which is set a vague number of years in the future (sounds like more than 20 years), Katniss and Peeta are a long-term couple and the parents of two young kids. Their children are innocent and the world, from their perspective, appears to be a much safer and happier place than the one Katniss and Peeta knew back when they were kids. The field the children play on is a huge cemetery – a relic of the Games and the war – but the children don't know that. To them, it's just some field.
Their children reveal Katniss and Peeta's hope for a better future. Katniss always has said that she'd never have kids because they would have to face horrors like the Hunger Games. Seems like she's mostly gotten over that fear, and that some of her faith in humanity has been restored, otherwise she never would have had children.
Katniss spent all three books of the trilogy playing some version of the Hunger Games. And the last lines of Mockingjay show that Katniss is still playing a game. Now, though, it's a game that reminds her of all the small, positive things in her world:
[…] I make a list in my head of every act of goodness I've seen someone do. It's like a game. Repetitive. Even a little tedious after more than twenty years.
But there are much worse games to play. (Epilogue.7-8)
This little game keeps Katniss positive and helps prevent her nightmares from taking over her mind. While she'll never escape the Games or her memories of them – and it seems unlikely that Peeta will either – at least the two of them can keep working to create a world where those kinds of events are just memories.