This symbol is called a 'trefoil' and visualizes radiation emitting from an atom. It must be displayed in places with radioactive materials, such as science labs or nuclear power plants.

Beyond actual warnings, it's often used in pop culture to metaphorically describe something as 'extremely powerful' or having a 'toxic charm.' For instance, you can use it wittily to describe very spicy food, an overwhelming person, or wildly popular music.