A showreel is a highly-edited series of footage that explain and detail what you, your portfolio, your services, or your products — usually, live footage is recorded by actors, musicians, etc. that showcases their talents and abilities, but showreels have expanded far beyond their original purpose. Now, all kinds of professionals and businesses leverage them.
It is also worth pointing out that you don’t even need to use live footage anymore. You can just as easily use 2D and 3D animations to create a professional showreel that shows you at your best.