What is a telephoto lens?
Obviously, it’s that big lens. The one that brings things close. Magnifies subjects in the viewfinder. Has a long focal length and a narrow field-of-view.
What are they for?
We use telephoto lenses whenever we cannot get close enough to our subjects either for practical physical or social reasons. Whether it is the Sun, the Moon; a lion or a bald eagle; the Superbowl or a wedding there are plenty of occasions when you can’t stand next to your subject.