Green leaf volatiles (commonly abbreviated as GLV) are volatile organic compounds that are released when plants suffer tissue damage. Specifically, they include aldehydes, esters, and alcohols of 6-carbon compounds released after wounding.
Some of these chemicals act as signalling compounds between either plants of the same species, of other species or even vastly different lifeforms like insects. Some, although not necessarily all, of these chemicals act essentially as plant pheromones.
The smell of a freshly mowed lawn is best known to humans, but for other forms of life GLV have a far less trivial function, mostly as a warning signal of oncoming causes of tissue damage, but also as a form of inter-species signalling - for example, to attract insects that prey on caterpillars that are consuming the plant.