Planck curve

All objects that have temperature emit electromagnetic radiation. Furthermore, the character of that radiation is determined by its wavelength (e.g. long wavelength radiation are radio waves, intermediate ones are microwaves, short ones are visible light, really short ones are X-rays, etc.)

It turns out that there's a relationship between the temperature of an object and the character, or wavelengths, of the radiation it emits. That relationship is: as an object gets hotter it emits shorter wavelengths at higher power.

An example of this is the unit on an electric stove (which is an approximate blackbody): When the unit is cool it's black. This is because most of the radiation it's emitting is long wavelength, which your eye doesn't perceive as visible light. As the unit heats up, it will begin to glow red. This is because, as the temperature increases, shorter wavelength light, which is visible to your eye, is being emitted at higher power.

In fact, if you could get the unit as hot as the Sun (you can't) it would glow white, with the same spectrum as the sun.