A die (plural: dice) is a device for generating random numbers, traditionally between $1$ and $6$.
Traditionally, opposite faces add to $7$, so:
- $1$ is opposite $6$
- $2$ is opposite $5$
- $3$ is opposite $4$.
The idea is that when you roll it, or throw it, or otherwise cause it to land upon a flattish surface, one of the sides will be face-up, and (with a good design of die) which side that is can not be precisely predicted.
Hence it serves as a generator of a discrete uniform distribution.