You need oak planks to craft certain furniture items in the game.
How Oak Planks Are Made
You can make oak planks by using the SAWMILL. Hover your cursor over the sawmill and press “F” to open. You will place up to 3 WOOD (the wood you get when you chop down the saplings) and coal from your inventory into the sawmill. The sawmill will produce 4 oak planks for every 1 wood block you place in it. Press “F” to retrieve the oak planks when it is done making them or if you have the sawmill open, just click on the planks when they are done and place more wood in there.