By default routing protocol like RIP and EIGRP summarize subnets into major classful network at classful boundary. In other word, these protocols perform an auto-summarization each time they crosses a border between two different major networks. To disable this behavior and advertise subnets, 'no auto-summary' command is used. Let's say router has two subnets 172.16.8.0/24 and 172.16.4.0/24 of Class B network and one subnet 10.2.0.0/16 of Class A. When auto-summary is enabled, router will advertise only summarized major classful network 172.16.0.0/16 for class B addresses into its Class A interface but you can change this default behavior and advertise both subnets using this 'no auto-summary'.
