Thread:Support/About MediaWiki:Securepasswords-valid/en/reply

It is indeed patchwork. This message is displayed to the user whenever they try to set a password (either upon registration or when changing passwords) to one that does not meet set requirements. $1 is the list of requirements a password must have, generated into a comma-separated list format via Language::listToText. Possible keys for inside the list are:
 * MediaWiki:Securepasswords-minlength/en ($1 in that message being a numeric minimum length for the password)
 * MediaWiki:Securepasswords-lowercase/en
 * MediaWiki:Securepasswords-uppercase/en
 * MediaWiki:Securepasswords-digit/en
 * MediaWiki:Securepasswords-special/en ($1 in that message being what is recognized as a "special character")
 * MediaWiki:Securepasswords-username/en ($1 in that message being the user's username, for the purpose of gender stuff)
 * MediaWiki:Securepasswords-word/en

An example message (completed) is below, for reference:
 * Your password is invalid or too short. It must: be at least 8 characters long, contain at least 1 lowercase letter, and contain at least 1 uppercase letter.

Hope that helps.