The numbers on golf balls serve a couple of purposes:
Identification: The most common number you'll see on a golf ball, usually ranging from 1-4, is simply there to help golfers identify their ball. For instance, if two players are both using Titleist Pro V1s on the same hole, one player can use a ball with a number 1 and the other can use a number 3 to distinguish their balls. Some manufacturers offer higher numbers, such as 5-8, for further personalization.
Model Number: Some golf balls may have a second, typically larger, number. This number indicates the model or type of the golf ball. For instance, Titleist's Pro V1 and Pro V1x have different numbers, so this number helps distinguish between these different models.
Compression Rating: On some older golf balls, you might see a number that indicates the ball's compression rating. This is less common on modern golf balls, but when present, it provides a rough indication of the golf ball's firmness. Higher numbers mean a firmer (harder) ball.
Handicap or Swing Speed Suggestions: While it's not standard, some brands use the number to suggest who should use the ball. For instance, a '90' might be used to suggest the ball is for players with a swing speed of 90 mph.
Remember to always mark your ball with a unique marking in addition to the number on the ball, especially in tournament play. This helps ensure you can positively identify your ball.