It means "it", as in the word "it". As in the test declaration reads like a sentence. You describe
an object by what it
does. Simple as that.
For example:
Bowling ball is round
Bowling ball has 3 holes
Might translate to a test hierarchy like this:
Bowling Ball
it is round
it has three holes
Which would translate to the following testing setup:
describe(BowlingBall, function() {
it('is round', function() {});
it('has three holes', function() {});
});
So because it reads well, it just becomes the way you separate individual test cases. It also encourages you to write your test description in a consistent manner because it
is part of the sentence that describes the test, which makes your test suite more readable over the long term.
In the end BDD is all about readability for the test writer. So this is simply sugar.