Focal Length (mm) is about how wide or narrow the field of view
is and how close the subject appears through the lens. The
shorter the focal length, the wider the angle of view (wide
angle). The longer the focal length, the narrower the angle of
view (telephoto). That also corresponds with how “zoomed in” the
image appears to you. A wide angle appears to be “zoomed out”,
and the a telephoto appears “zoomed in”. Simple. Wide lenses are
low in millimeters, i.e. 20(mm). Narrow lenses are high in
millimeters, i.e. 200(mm). The range is enormous. 50mm is
considered standard and similar to the way the human eye sees.
Physically, Wide angle lenses are short and telephoto lenses are
long. i.e. photographers you see at the side of a football match
have massive lens as there are getting close shots of the action
from far away. A wide angle lens is very short. Wide angle lens
allow you to stand close to the subject and still maintain a
wide view. Telephoto lens bring things closer to you. Macro
Lenses are for close ups.