It checks these directories when you log in, then remembers all the files in them. CERI accounts are generally set so that standard places for executables are already in your path. You need the rehash to tell UNIX to update its list of executables for you. If you don't want to go through all that and don't mind typing, you can always type the path and filename.
Environments are handy things that many programs use.
There are tons of them. Somewhere this environment was probably already defined for you. You can see all your environments by typing printenv. But, if it works for the person sitting next to you and doesn't work for you, odds are it is some path or another or some environment or another that is different. Either that or the computer hates you probably because you tend to smudge the computer screen with your greasy fingers.