Pretty straight forward. Use one to set and environmental variable, and one to unset and environmental variable! To get all the environmental variables you can use printenv
This command deletes an environment variable from memory and also
removes it from the NVRAM device (if present).
setenv varname value
This command sets an environment variable. By default, an environment variable
is stored only in memory and will not be retained across system restart.
if an NVRAM device is present, the variable is also stored there and
is persistent across reboot.