A hatch in fly fishing is a mass emergence of a particular insect type, usually mayflies, caddis flies, midges or stone flies.
A hatch sees hundreds of thousands or even millions of aquatic insects leave the river bed as nymphs in a particular area and ascend to the surface where they become emerge through the water’s surface and eventually take flight.
A hatch brings trout to the surface to feed on the hatching insects.
When there is a hatch occurring, you will often need to fish with a fly that closely resembles the hatching insect or go home empty handed. This gives rise to the term “match the hatch”.