Definition - What does Ionosphere mean?
The ionosphere is an area in the earth's atmosphere which is between the exosphere and the stratosphere. It consists of three layers, which all have the same characteristic of high concentrations of ionized particles. These particles are ionized by radiation from the sun. The ionosphere extends from approximately 50 to 250 miles (80 to 400 km) above the earth's surface.
Safeopedia explains Ionosphere
Ions are atoms with positive or negative electrical charge. The ionosphere is important in radio communication, especially over great distances. This is because it can reflect radio waves from the earth's surface back down. Depending on the frequency of microwaves it will either allow them to pass through or reflect them away. The threshold for this is approximately 100MHz so anything above this passes through and anything below is reflected.