What is the subnet mask used for?

It is used to tell the machine who it can talk to without the assistance of a gateway.

192.168.1.1/24 or 192.168.1.1 255.255.255.0

Can talk to anyone with the IP of 192.168.1.2-255 withoutthe use of a gateway.

Any requested addressed to someone outside of that gateway will automatically be sent to the gateway for processing.