Focal Length is about a lens's viewing perspective and is expressed in millimeters (mm).

I photographed these examples several years ago, using 2 different cameras and multiple lenses to cover the entire focal range seen here.

Watch the tombstone in the center and notice how our perspective changes from one photograph to another.

Normal  human viewing perspective has always been considered to be roughly 50mm.

Although some would argue that technically  with digital cameras it's really between 38mm and 43mm, I'm still going with 50mm.

Anything lower than 50mm pushes objects away  from us, opening up our field of vision (wide-angle and fisheye).

While going over 50mm brings objects closer  to us (telephoto and telescopes).

"If your photos aren't good enough, then you're not close enough."

Robert Capa