In short. User-generated content: dungeons, mini-games and housing made by players using blocks.

Quoted from cinder's blog:

Dungeons Players can use the block system to create dungeons, place traps, monsters, conditions and many more options, just like a real developer. These dungeons can be played by other players as well, or the creators can find friends and conquer them. Sounds really awesome!

Housing There is player housing system! Created using blocks, the price of each block is according to the region and quality of material. Halfway creating your dream MapleStory 2 house and ran out of money? You can co-create houses with other players and have several co-tenants under 1 roof! Decorations can be created as well.

Mini-games Somewhat similar to making dungeons, not much detailed information since this is written together with the dungeon function. Stay tuned for more info!