Elden Ring is yet another game where you have to memorize particular items and what they are used for. Since there are lots of important items, you don't want to accidentally throw the wrong ones away. This is what Stonesword keys are for in Elden Ring.
Stonesword keys are used on Imp Statues to open barriers, which often hold a rare weapon or item that is very valuable. The Stonesword keys themselves are also quite rare, so you will want to keep an eye out for them at all times.
You can find them just about anywhere—in chests, after defeating enemies, and being sold by certain merchants. There isn't a place that we can definitively tell you will not house a Stonesword key.
Related: What are Evergoals in Elden Ring?
Some Imp Statues will require two keys in order to be activated, so you may want to hold a few in your inventory when you find them, then backtrack to the statues to see where you need one and where you need two.
You will most likely encounter your first Imp in the first 30 minutes of the game, but you will be keyless at this juncture. Make a mental note of its placement so you can return later.
Looking for more help? Why not check out How to upgrade the Sacred Flasks in Elden Ring on Pro Game Guides?