It's the number of bristles on the brush - not how hard they are. The larger the number, the bigger the toothbrush. Aim for a smaller number, so you can get the toothbrush into all the spaces in your mouth. The larger the bristle part of the toothbrush is, the harder it is to clean your teeth. Always get a soft or medium brush - the hard bristles aren't as effective at getting between teeth and the gumline.