Certain artifacts in Minecraft Dungeons require you to use souls. You acquire souls by taking out enemies during your playthrough. After an enemy drops, you should see a small ghost emerging from where they fell, and watch as they float over to your character. While soul-based artifacts do have a cooldown attached to them, they also require souls for you to activate them.
You can see how many souls you have directly underneath your artifacts tab on the bottom left. There’s a small ghost icon underneath this tab and a bar. On the left side of the bar should be a little white skeletal-like face, which stands for the souls you have on your character. The more souls you have, the more this bar grows, and the field fills in a bright blue color. You won’t be able to see how many souls you have acquired, but it does indicate how many you have.
A majority of the artifacts take about a third of this bar to activate, likely because you can have three artifacts simultaneously on your character, and they all could potentially use souls. Other items in the game make it easier to acquire souls during a run, such as the soul bow, or the phantom armor. These increase the number of souls you receive per kill, making it easier to fill up the soul bar.
These soul-based artifacts are compelling, like the Soul Healer, which acts like a second health potion. They can turn the tide of a fight and ensure you live to see the end of a level, but they do require you to be more aggressive. You can’t wait for their cooldown to arrive and then spam the artifact. You need to aggressively go after enemy mobs, and reap their souls for your use.