Use comments to document your scripts. Leave yourself notes about what variables are for and what each section of code is doing. This will make it much easier to maintain the code later.

Everything on a line after a double-slash (//) is considered a comment and ignored by MEL (except inside strings, of course).

// This is a comment.
print(5 + 10); // This is a comment too.

You can comment out an arbitrary block of code by surrounding it with/* and */.

/*
This is a multi-line comment.
You can type as much text in here as you want.
*/

Freeform comments cannot be nested:

/*
This is a comment.
/* Sorry, you can't put a comment inside a comment. */
You'll get a syntax error here.
*/

This type of comment doesn’t work in animation expressions. Only // works in expressions.