The focal length of a lens is not the lens’ physical dimension and has little to do with its overall size. The focal length of a lens is an optical property of the lens and measures the distance, typically in millimeters (mm), between the optical center of the lens and the camera’s sensor when the camera is focused to infinity (∞). Now you can forget this definition, as you will never need to understand it to use your camera. However, lenses are designated by their focal length, and is typically shown on the barrel of the lens. For example, a 70 – 200 mm lens has a focal length between 70 and 200 mm.
Now to a more useful understanding of what focal length means to a photographer. Focal length describes the angle of view of a lens and how large subjects appear in the frame. The longer the focal length of a lens, the narrower its angle of view as shown in the figure below. Subjects appear larger using long focal length lenses than they do viewing them with only with our eyes (your eyes have a focal length of around 43 mm). Lenses with short focal lengths take in a much wider angle of view and subjects seem much smaller in the frame than they do to our eye. For an excellent graphic explaining focal length, please see: https://capturetheatlas.com/wp-content/uploads/2020/04/focal-length-and-angle-of-view.jpg.webp.
What is the best focal length for taking pictures of birds? It depends on the bird. If the bird is comfortable around you, due to feeding or naturally has little fear of humans, you can use lenses with smaller focal lengths (e.g., standard focal length lenses). For example, I used my cellphone with a focal length of around 35 mm to photograph Little Penguins, who appeared to have little fear of humans. Most birds are more fearful and typically require larger focal lengths (i.e., telephoto and super telephoto lenses).
What is zoom? Many cameras are marked with an optical “zoom” value, which is a useless value when comparing different camera models. The zoom value is calculated by dividing the lens largest focal length by its smallest focal length. Therefore, to use this value to compare lens or cameras also requires the knowledge of the minimum focal length for each. For example, a 200 – 500 mm lens has a zoom value of 2.5 while the Canon PowerShot SX540 HS has a focal length of 4.3 – 215 mm with a zoom value of 50. So, at first glance, which one appears to show the subject the largest? The 200 – 500 mm lens? Well, it depends on the size of the sensor in the camera.