"That portal you found is why we've summoned you, Guardian. I suspect it was placed there by the Great Evil. At some point, you'll probably want to make a home for yourself, somewhere in the wilderness. To prove you're capable of building shelter, I want you to place 20 Spruce wood planks in the wild."
When you accept this quest, go back to /warp wild and type /random to teleport to a random location within the wild and search for Spruce wood. Spruce wood is mainly found in snow biomes. Use your axe to cut down at least five Spruce wood blocks and use the 4x4 crafting space in your inventory to turn them into planks. You can use /random again to teleport to another location until you find your ideal place to live, or you could join someone's town. The quest will complete when you place those planks.
You can continue to build your house, or you can do so later if you want to complete the rest of the quests. If you want to return to the planks you placed, use /sethome before returning to King Xero's throne. Use /home when you want to go back.