This template does nothing.
More exactly, it outputs the null string. It's useful for programming advanced templates.
It often is used within a template to the right of an equals sign.
Alternatives
It's also possible to use {{ns:0}} instead of {{void}}. Attackers could manipulate {{void}} but not {{ ns:0 }}, the empty/void/null string for the main namespace.
Some magic words like __END__ might also have the desired effect of doing nothing.