In modal logic, □ is the necessity operator.1
It is necessary that P.
For P to be necessary is for P to be true in all possible worlds. Compare □P with the modal sentence ◊P, which means P is true only in some possible world (possibly P).
□ can be formally defined using ◊, which is the modal possibility operator, and the negation operator ¬ from classical logic.
□P = ¬◊¬P
Necessarily P = Not possibly not P
For those who are dubious of their browser's character set, □ is indeed meant to display as a box.