Focal length is the distance over which the telescope brings light to a focus, and with aperture it sets the focal ratio, which is the number written as f/5 or f/10. Together they decide magnification per eyepiece, field of view and how forgiving the instrument is.
Focal length sets magnification per eyepiece
Magnification is the telescope's focal length divided by the eyepiece's. A 300mm instrument with a 20mm eyepiece gives fifteen times; a 1200mm instrument with the same eyepiece gives sixty. That is why a long instrument feels high powered with ordinary eyepieces and a short one needs short eyepieces or a barlow to reach the same magnification.
Focal ratio sets the character
Dividing focal length by aperture gives the focal ratio. Fast instruments, around f/4 to f/6, give wide fields, suit sweeping and photography, and demand better eyepieces because they bend light more steeply. Slow instruments, f/8 to f/12, give narrower higher power views, suit planets and double stars, and are gentler on cheap eyepieces.
What this means when you compare two prices
Celestron's $79.95 instrument is 76mm at 300mm focal length, which is f/4: short, wide and fast. A 130mm tabletop dobsonian is typically f/5. An apochromatic refractor at $498.00 might be f/6 and built for a camera. Two instruments at similar prices can be aimed at completely different jobs, and the focal ratio is where you read that.
Questions people ask about telescope
What is telescope focal length?
The distance from the main optic to the point where it forms an image, in millimetres. It sets the magnification each eyepiece gives.
What does f/5 mean on a telescope?
The focal ratio: focal length divided by aperture. A 500mm focal length on a 100mm aperture is f/5. Lower numbers mean wider, faster instruments; higher numbers mean narrower, higher power ones.
Which focal ratio is best?
Neither, for looking. Fast is better for wide fields and photography, slow is better for planets and is more forgiving of cheap eyepieces. Aperture matters more than either.