To swoop is to move downward quickly and dramatically, like an eagle that swoops down to catch its prey.
A swoop is an impressive movement best demonstrated by a bird. If a bird is high in the air, then moves down quickly, making an arc or loop, that's an example of a swoop. People can also swoop — three muggers might swoop in on a victim, for example. The phrase "one fell swoop" means "all at once," and Shakespeare used it first, in "Macbeth:" "What, All my pretty Chickens, and their Dam, At one fell swoop?"