This relates to the millimeter (mm) or “range” of your camera lens. For example, if you have a lens with a “zoom” this zoom range allows for taking pictures at different focal lengths.
Let’s pretend you have a 24mm to 120mm “zoom” lens. If you are shooting with the widest angle of view, so with no zoom, you have a focal length of 24mm. Taking a photo with the other extreme of the zoom range and your focal length now becomes 120mm.