Options: Security

Encrypt macro - encrypt or decrypt current item's text. Decrypting requires the same password that was used when encrypting.

If "All containing text" is checked, instead encrypts or decrypts all items that contain the text. The text in items must be a full comments line, ie a line that begins with a space or semicolon. Don't include the space/semicolon. When decrypting, decrypts only items that are encrypted using the same password.
Note 1: If you encrypt a macro that contains a dialog definition, the dialog will not work, unless the dialog definition is assigned to a variable which is passed to ShowDialog.
Note 2: When you click function name in code editor and press F1, you can see its parameters and help section (comments above code), even if the function is encrypted.
Note 3: By default, QM creates backup files. They may contain unencrypted (older) versions of your encrypted macros. The bad is that somebody can see the macros if you don't delete the files. The good is that sometimes there you can find unencrypted versions of your encrypted macros in case you have lost password. Usually QM eventually deletes the files. Look in Options -> Files.

Encrypt the password to use with a function - encrypt a password for using with a function that supports encrypted password. Read more.