A swaption strangle is a predefined strategy constructed from payer and receiver swaptions.
With a strangle you can do either of the following:
Buy a receiver swaption and buy a payer swaption both with the same expiry but different strike prices and notionals.
Sell a receiver swaption and sell a payer swaption both with the same expiry but different strike prices and notionals.
For more information on a swaption (and how to price it and what points to consider) see Swaption.
Advantages of a Swaption Strangle 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 swaption strangle. The investor will receive a payout if on expiry the underlying swap settles beyond one of the two strikes. 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.
Because the contracts are usually purchased out of the money, strangles are usually cheaper than straddles.