Berry Leather is a useful resource that can be derived from Berries. These are only found in a specific location, so you will need to brave the wilds to obtain these if you're looking to craft with it. Make sure to equip yourself with a bow and some good armor so you don't find yourself running back to your loot!
To get Berry Leather in Grounded, you will need Berry Chunks. You can get these by harvesting Berries, which you can only find by heading to the southeastern portion of the map where the berry bush is located.
Once you reach the big bush, you will see big blue berries hanging from the branches. You can find some on the ground as well, but if you have a bow you can just shoot them down. Take out your axe and chop one up. You will get a Berry Chunk from it, which you will now need to take to a Field Station. Lucky for you there's a Field Station right in the big bush, so you can do the research there if you'd like to!
Be very careful in this area because it's a prime location for Orb Weaver Spiders to spawn. Try to avoid them as much as possible unless you need some spider ingredients for whatever you're looking to create.
Once you've done your research, you can then make Berry Leather out of the chunks once you can access a Workbench. You get one leather per three chunks, so make sure to gather quite a bit of this stuff so you don't have to head back to this location.
Berry Leather is used in quite a few armor recipes, which is one of the main attractions of it. You will also need it for the Insect Hammer, which is the upgraded version of the hammer that can be used to break more advanced materials!