Minify, Mangle and Compress
Minification means that you remove all unnecessary characters without changing the functionality. These unnecessary characters usually include white space characters, new line characters, comments, and sometimes block delimiters, which are used to add readability to the code but are not required for it to execute.
Mangling your script means renaming your local variables to (usually) single letters.
Compressing means to optimize your script like joining variable declarations, optimizing if-statements, discard unused variables or joining simple statements using a comma operator.