It is a kind of preferred stock where the dividends issued will change with a benchmark, most often a T-bill rate. The price of the dividend from the preferred share is set by a fixed formula to move with rates, and due to this flexibility preferred prices are generally more stable then fixed-rate preferred stocks.
The preferred category of stocks is more secure as they would be one of the first of the equity holders to receive dividend payments in the event of the company's liquidation. There is generally a limit to the amount the rate can vary on the dividend, adding extra security to the issue.