The environment is a set of named strings every program starts with. It is how configuration reaches a program without arguments, and it is also why the same command behaves differently on two machines. Reading it is how you find out which one you are on.
About 1 min of reading
Scope a variable to one command.
Set for one run
Before you run it, say out loud what it will print. Then open the answer.
Prove it did not leak
Before you run it, say out loud what it will print. Then open the answer.
Configuration that differs per machine goes in the environment, not in a file in the repository. A secret committed to a file is visible to every reader of the repository, and the environment is the ordinary place it belongs.
A program that expects a variable and gets nothing usually fails with a message that names the variable. That failure is the design working; the fix is to set it, not to change the program.
echo "$PATH" prints a value that decides where programs are found. Printing a variable before using it is the fastest way to see what a program will actually do.
PORT=3000 node server.js sets the variable for that one run only. That scoping is the clean way to change behaviour without leaving the shell in a state you forgot about.
A plain assignment is local to the shell; export sends it to programs the shell starts. The difference explains why a variable you set works on the terminal and not inside a script.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.