Question : Disable Auto-Summary on Router

If I add:
router eigrp 109
no auto-summary

What exactly does it do?

I think that it advertises the mask along with the prefix.

Not assume Class A 8 bit mask, Class B 16 bit mask and Class C a 24 bit mask.

If your network is non-contiguous and not completely hierarchical then you have to disable auto-summary.

I read something about EIGRP having problems turning off auto-summary if you are using both private address space 10 and 192.168?

Also the documentation for EIGRP says that it "automatically summarizes at classful network boundaries". What exactly does that statement mean?

Answer : Disable Auto-Summary on Router

By default, EIGRP will summarize routes using the classful boundary when the route is advertised over a different network. For example, advertising the 172.16.1.0/24 network out an interface with an address of 172.17.1.1/24 will be advertised as the 172.16.0.0/16 network.

However, if auto-summary is disabled then the route will be advertised as the 172.16.1.0/24 network.