President Snow said in the announcement that the tributes would be chosen from among prior victors of the Games. Katniss is returning to the arena since she is the only female District 12 winner. Peeta also returns because he is the only male victor from District 11.
This means that both of them will be forced to go back into the Games and try to win again. There's a good chance that they'll both be killed during the events, but if they survive then they could win it all again.
Katniss already went through this in Mockingjay - Part 1, where she returned as a hologram to announce that Primrose had been selected as the new winner of the 74th Hunger Game. Although nothing much happened in the end since there was no actual Games involved.
In Catching Fire, we're being told that this time around things are going to be different. That President Snow has changed his mind about winners not being allowed to win again. He's making an example of these two by forcing them to go through the motions once more even though they're supposed to be dead.
Primrose also returned as a hologram at the end of Catching Fire to tell us that Katniss and Peeta have been chosen for another victory tour.
When Katniss destroys the force field, the story concludes. The hovercraft transports Beetee and Katniss, along with Finnick and Haymitch. Peeta and Johanna were driven to the Capitol by Snow. Gale informs Katniss that District 12 has been destroyed and that they are in route to District 13. As they fly over District 13, it is revealed that much of it has been burned down by an angry crowd who have taken out their anger on their surroundings.
During the ceremony at the end of the games, when Katniss steps up onto the podium, she is wearing a dress that is completely covered in blood. This is because during her final confrontation with President Snow, he ordered his men to kill everyone in District 13 so that no witnesses remained. When Katniss throws her hat into the audience, it catches fire.
In order for someone to be truly successful at hunting people, you need to know exactly what you're doing. You need to study them, understand their habits, and find a way to take them out quickly and quietly. Once you catch fire, there's no putting out the flames until they burn out on their own. And since there's no way to tell how long this might take, it's important to estimate how many people you can save with your limited resources. In this case, Katniss had only one sparkler available to her.
President Snow reminds Katniss that it was President Coin, not President Snow, who put off the explosives that murdered her sister. As a result, a riot breaks out, and President Snow is discovered dead, either from choking on his own blood or being crushed by the throng. This all happens during the 75th Hunger Games.
Now, this doesn't necessarily mean that President Snow is immortal - he could have died from natural causes or been killed by another contestant. But since he was delayed by his death, Katniss has an extra day to think about District 12 and their situation. This gives her time to come up with an idea that may help them both.
In the final chapter of Mockingjay - Part 2, we find out what happened after President Snow's death. Knowing that Katniss has one more day before she has to make a choice for herself, Peeta goes back in time and saves her life. This allows Katniss to live long enough to see Capitol president Coin take office instead of president Snow. In addition, Peeta also prevents Katniss from killing herself by shooting her with a stun bullet, giving them both a chance to change their minds.
So President Snow did die near the end of Mockingjay - Part 2, but not before saving Katniss' life.
President Snow is prepared to go to any length to keep Katniss under his control. He threatens to murder Gale unless she proves to him and the rest of the nation that she and Peeta are in love. He is hoping that this demonstration of devotion would put an end to the uprisings that have begun in several of the districts. Instead, it sparks her own rebellion.
Snow believes that if anyone else knew about their relationship, they would try to hide it or even use it as a weapon against him. So he comes up with a plan to have them marry in front of the Capitol audience so everyone can see how much they care about each other. Then, he intends to have them both killed on the same day so no one can say that he isn't showing mercy.
Katniss has always been honest and forthright. She thinks that no one deserves to be punished without knowing why they are being punished and what they have done. Even though she knows that this marriage is a trap, she goes along with it because she wants to save Gale's life. She believes that if she can prove to President Snow that she loves him more than anything else, he will spare their lives.
Katniss and Gale get married at noon before an audience of hundreds of people. After the ceremony, President Snow orders that they fly to District 12 immediately for a public tribute ceremony where Katniss' sister Primrose and some of the other victors will speak on her behalf.