Along your adventure, you may spot glowing human skulls scattered throughout the land. Definitely don’t ignore these. As soon as you see one, smash it open. This can be done with your weapon, running it over with your spectral steed, or even just rolling on it. Inside each of them are Golden Runes. These can be sold at shops in exchange for actual Runes, which are crucial for leveling up and buying more items and techniques. You’ll most certainly want to keep an eye out for these.
Source: https://www.distractify.com/p/elden-ring-tips-and-tricks