In young children their Eustachian tubes are not as well developed as adults resulting in accumulation of fluid in the middle ear space. The two main complications that occur because of this is less hearing as the ear drum cannot move because it is impeded by fluid and thus there is a delay in speech.
The other complication is that a warm moist environment is ideal for bacteria to grow and thus each infections are more common.
A grommet looks like a button with a hole in the middle of it which is placed through the ear drum thus creating an opening through which the fluid can drain. It thus temporarily takes over the function of the Eustachian tube.
After a few weeks to months once the Eustachian tube has healed and is able to function normally the grommet is naturally extruded into the ear canal. Although it is a minor procedure a number of complications can occur such as bleeding, infection perforation, re insertion or even surgical removal.