|The hair of the bow grips the rosin, which in turn grips the string. In an effort to get a better sound, many new players frantically rub the rosin along the bow. This can cause quite a lot of damage as it heats up the rosin - which doesn't need much encouragement to form a hard, glassy surface which coats the hair of the bow. When this happens, the bow will slide over the string and won't grip it. The temptation is then to put more rosin on, which is unlikely to help. The only cure if this happens is to rehair the bow - not an expensive job but not necessary if you are careful to start with.
To apply rosin, just stroke it carefully along the bow hair once or twice. That's enough to rosin the bow properly.