Focal length is the distance from the lens's optical centre to the sensor when focused at infinity. Short numbers mean wide views: 16mm takes in a whole room. Long numbers mean narrow views: 200mm isolates a face across a field. The number is a physical fact about the glass, not a setting.
Beyond field of view, focal length changes the apparent relationship between near and far. Wide lenses exaggerate distance — a nose looks larger, a hallway looks longer. Long lenses compress it, stacking a subject against a background that seems to loom right behind them. This is why portraits are traditionally shot between 50mm and 135mm: it flatters facial proportions.
Worth separating from perspective, which is set only by where you stand. If you swap a wide lens for a long one without moving, you have cropped. If you move backwards to keep the subject the same size, you have genuinely changed how the image reads. Photographers call this lens compression, and it is really foot compression.