H II regions are vast clouds of ionized hydrogen gas, where new stars are born. The clouds of the gas are being pushed by shock waves caused by supernovae and other mechanisms, which can trigger a collapse and new stars being born during this process.
Most of the observable H II regions are located in our galaxy Milky Way, but there are also visible some H II regions in our neighboring galaxies, for example in M33 Triangulum Galaxy.
The most famous H II regions are M42 Orion Nebula, M16 Eagle Nebula.