Callable stock is an ownership interest (shares) in a corporation that can be "called in" by the corporation at a specified price.
For example, a corporation might issue 9% $100 Preferred Stock. The stock agreement (indenture) states that the stock is callable by the corporation after three years at $109 per share plus any accrued interest. If in the fourth year, market rates decline to say 7%, the corporation can call in the preferred stock by paying the call price of $109 plus any accrued interest.
The callable feature allows the corporation to get out of the preferred stock agreement requiring it to pay the $9 per share dividend. In turn, the stockholders will be deprived of receiving the $9 dividend in a 7% market. The call price has the effect of limiting how high the market value of preferred stock will rise.