CIsco devices store commands in a configuration file. Immediately after you type a command in the global configuration mode it is stored in the running configuration. A running configuration resides in a device’s RAM, so if a device loses power, all configured commands will be lost.
To avoid that scenario, you need to copy your current configuration into a startup configuration. A startup configuration is stored in the nonvolatile memory of a device, which means that all configuration changes are saved even if the device loses power.
To copy your running configuration into the startup configuration you need to type the command copy running-configuration startup-configuration.