It generally means introducing a predator species which is supposed to prey on the invasive species causing a problem. It is using a "biological solution" to tackle a biological problem. Unfortunately, this doesn't always work, as illustrated in Hawaii's snail problem. Way back in 1936, when people basically weren't thinking about these things, gardeners in Hawaii deliberately introduced the giant African snail (Achatina fulica Bowdich) as "decoration". These giant snails took one look at all the tasty local vegetation set in a lovely warm, moist climate, and promptly multiplied out of control. In 1955, the Hawaii State Department of Agriculture introduced the even larger snail Euglandina rosea, as a known predator of other snails, to try to control A. fulica. However, E. rosea apparently decided they couldn't be bothered with A. fulica when there were much tastier things around, and instead have been cheerfully eating their way through Hawaii's native populations of smaller snails and slugs. See http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Euglandina_rosea.html for details of the story, and further links on invasive species.