A swaption straddle is a predefined strategy constructed from payer and receiver swaptions.
With a straddle you can do either of the following:
Buy both a receiver swaption and a payer swaption with the same strike price, expiry and amount.
Sell both a receiver swaption and a payer swaption with the same strike price, expiry and amount.
For more information on a swaption (and how to price it and what points to consider) see Swaption.
Advantages of a Swaption Straddle Strategy
An investor who believes that a swap rate will change significantly, but does not know in which direction it will change, may buy a straddle. The investor will receive a payout if the swap rate expires at a different rate from the strike, in either direction. If the swap rate does not change significantly, the investor will experience a loss since the payoff of the swaption exercised will not cover the combined premiums of the two swaptions needed to execute the strategy.