Transports are air units capable of carrying ground units.
Transports can carry units quickly across obstacles. They can be used to sneak around defenses like protoss photon cannons, terran bunkers, or zerg spine crawlers, or to infiltrate the rear of a base or harry workers near a mineral line.
Travel to Islands
Islands are typically only accessible by transport (or by protoss warp gates, stalker blink, or zerg nydus worms). With a transport, you can bring an army to invade an island or send a worker to build an isolated expansion.