This estimate may not be very accurate, but it is certainly a reasonable one in view of the fact that stellar interiors and atmospheres, gaseous nebula, and much of the interstellar hydrogen are plasmas. The closest example of a plasma to you is probably a fluorescent lamp, where mercury vapor is ionized by electric current passing through it, or a neon sign. Other everyday examples of plasmas are the flash of a lightning bolt, the soft glow of the Aurora Borealis, and a slight amount of ionization in a rocket exhaust. Above our heads, the atmosphere begins to be ionized at a few tens of kilometers altitude and becomes the ionosphere, which we use to bounce radio signals off of for short wave communication.
Matter exists in three forms, solid, fluid and plasma.
The fluid form, in turn, being divided into the categories of liquids and gases. The solid and fluid states are well known from everyday experience. However, if one adds even more heat to a gas, the electrons are ripped away from the atoms. The result is an ionized gas consisting of negatively charged electrons and positively charged nuclei. The solar wind for instance is almost completely composed of electrons and protons, i.e. ionized hydrogen.