Answer
The number of possible states increases.
Work Step by Step
Entropy usually refers to the idea that eventually everything in the universe moves from order to disorder. Entropy is the measurement of the change from order to disorder (or the other way around).
The more disorder, the higher the entropy will be.
When a gas expands into a vacuum, then the volume increases, which will allow more disorder, because the number of possible states increases (such as where the particle is located).
More disorder corresponds with higher entropy and thus the entropy will increase.
