A blackboard bold font style commonly used to denote standard number systems or important mathematical sets.
Examples
Common number sets in mathematics
\mathbb{R}, \mathbb{Z}, \mathbb{Q}, \mathbb{N}, \mathbb{C}
Function mapping between number sets
f: \mathbb{R} \to \mathbb{C}
Set theory with blackboard bold notation
x \in \mathbb{R} \text{ and } y \in \mathbb{Z}