The ultrasonic Mosquito repellent application emit a very unique high frequency sound (ultra sound) that most mosquitoes find distasteful and prevents you from getting bitten by mosquitoes.
The pitch of the sound is so high that most humans will not notice anything.
The problem is that the idea isn't new or untested. In fact, ultrasonic repellants that claim to work on the same principal have been around for decades. Before smart phone apps, the sound was emitted from plug-in or battery-powered electronic devices. Those are still on the market too. Radio stations have even tried broadcasting the sound. So the devices have been tested numerous times, in various forms. And guess what? They fail the tests. Sometimes, they even attract more mosquitoes.
Ultra-sonic repellants usually make one of two claims: either it's the noise of dragonfly wings (or sometimes bats), which frighten off mosquitoes because dragonflies and bats are mosquito predators. Or it's the sound of a male mosquito calling for a mate, which will keep biting females away because "once a female mosquito has her eggs fertilized she will actively avoid any further contact with the male," as this app explains.
You can read reviews and decide for yourself whether the devices are actually working for those who say they do, or whether it's coincidence, or whether they've stumbled on some perfect combination that has eluded testers.