What is the 'greenshoe' option that expanded SpaceX's IPO to $85.7 billion?

SpaceX said its underwriters exercised the 'greenshoe' option to purchase additional shares, increasing total proceeds from its IPO to $85.7 billion from $75 billion. The 'greenshoe' is ​a standard feature of most US stock market listings that acts as a safety ​valve. It helps underwriters support the stock and limit sharp price swings after trading begins due to strong demand.