A supergiant is an old large star. Supergiants are rare and short-lived, but super bright in terms of stars that we can see from Earth. Because they are large and bright, you can spot them at night with your naked eye and I wrote how you can see them in the daytime through a telescope, if you know where to look.
They range in temperature from about 3,450 K to over 20,000 K (red to blue spectrum).
Betelgeuse and Antares are two red supergiants. Betelgeuse of the Orion constellation is 1000 times larger than our Sun and 14,000 times brighter.
Rigel, in the constellation Orion, and Deneb, in Cygnus, are two blueish-white supergiants. The following chart gives you a idea of their brightness and their spectral class compared to other stars in space.