From this page of the Valheim guide, you will learn how to obtain the Ancient bark, which is a very valuable resource.
The Ancient bark is a type of wood you can find in Valheim. Before you can go to a specific biome and start cutting down the right trees, you need to create a Bronze Axe - tools made of wood or stone will not work.
The Ancient bark is obtained after cutting down the Ancient trees which can be found in the Swamps. It should be noted here that the Ancient bark is not the only resource you get after cutting down the Ancient trees - you also get regular wood.