BTS has finally revealed what the project is. The project is actually a collaboration with UNICEF, and is aimed to "stage campaigns against violence toward children and teens around the world, with the hope of making the world a better place through music."
Over the next two years, BTS will be working with UNICEF in the following ways: donations worth KRW 500 million from Big Hit Entertainment and the seven BTS members, donations of 3% of the income from the sales of physical albums of the Love Yourself series, donations of 100% of the income from the sales of official goods for the LOVE MYSELF campaign, and donations at the donation desks installed by UNICEF. Fans can also use the hashtag #ENDviolence to help.
You can read more about it here.