It was mentioned about half-way through the article.
> And finally, in many high valued startups where there are hungry investors, the founders get to sell parts of their vested shares at each round of funding. (At times this opportunity is offered to all employees in a “secondary” offering.) A “secondary” usually (though not always) happens when the startup has achieved significant revenue or traction and is seen as a “leader” in their market space, on the way to an IPO or a major sale.
> And finally, in many high valued startups where there are hungry investors, the founders get to sell parts of their vested shares at each round of funding. (At times this opportunity is offered to all employees in a “secondary” offering.) A “secondary” usually (though not always) happens when the startup has achieved significant revenue or traction and is seen as a “leader” in their market space, on the way to an IPO or a major sale.