The ionosphere is not a layer in the atmosphere. It is a part of the thermosphere, the top of the mesosphere, and the bottom of the exosphere. The ionosphere is the part of Earth’s atmosphere that is full of ions.
When radiation from the Sun hits gas particles in the atmosphere, the energy from the radiation knocks electrons off of the gas particles. This creates negative and positive ions. These ions bounce around the ionosphere.
The ionosphere is important because it allows radio signals to move all over Earth. The radio signal travels up to the ionosphere and then reflects back down to Earth. This allows radio signals to be shared all over the Earth.
We can see the effects of the ionosphere on Earth in the form of the Aurora borealis and the Aurora australis. When ions are hit by radiation from the Sun, they absorb too much energy. The extra energy is then released in the form of light. This light causes the light shows called the Aurora borealis (in the north) and the Aurora australis (in the south).