The characters in my campaign are getting to higher levels now. In the PHB, under the base attack ratings, it lists +20/+15/+10/+5 and such at the higher levels.

I understood it when it was just one number, but what do these multiple numbers mean?

FYI= The above numbers are for a lvl 20 barbarian.

It means that he/she has four attacks per round with base attack +20 for the first, +15 for the second and so on. If you have the PHB try page 22 for further explanation.

Sweet, thank you.