Scale is simply the ratio between the model and the real car. A 1:24 model is one twenty-fourth the size of the original, so a 4.8 metre saloon becomes a 20 centimetre model.
Smaller numbers mean bigger models. 1:18 is display-cabinet size with opening panels and detailed engines. 1:24 is the sweet spot for younger collectors — substantial in the hand, but still affordable.
1:64 is the pocket-money scale. It is the one that ends up in a school bag, and the one most likely to survive being sat on.