Using Environment Variables
This page is assuming you use bash or another shell in the Bourne family of shells. Most of the informtation will be correct for other shells as well, but the examples cannot be assumed to be.
HPC2N recommends using bash, since it is the only shell that is fully supported by the batch system.
Environment variables are a set of dynamic named values, generally meant to make your life easier. Some are built-in, some are set by loading various modules, and you can also define extra to help yourself.