It means the replacement of a unit of plaintext (i.e., a meaningful word or phrase) with a code word (for example, "wallaby" replaces "attack at dawn").
Some accurate throws of up to 660 feet (200 m) have been recorded, and it would have been easy to get within that distance of an unsuspecting kangaroo or wallaby.
Moves are supported for the reintroduction of locally extinct animals such as malleefowl, common brushtail possum, rufous hare-wallaby or mala, bilby, burrowing bettong, and the black-flanked rock-wallaby.
One wallaby was chased by dogs for 500 metres and suddenly doubled back and came back within 6 metres of John Gould and leapt over his head; it could jump 1.8 metres high.