That question has a lot of answers. They can be required to cover within 10 days or they can naked short and not have to cover unless pressure and the price rise and forces margin calls.
A measurement of a company's issued shares that are currently shorted, expressed as the number of days required to close out all of the short positions.
For example, if a company has average daily volume of 1 million shares and 2 million shares are currently short sold, the shares have a cover rate of 2 days (2M/1M).