From the Super Mario Wiki
Mario rescues a Mushroom Retainer in Super Mario Bros..
“Thank you Mario! But our princess is in another castle!”
—Toad, Super Mario Bros.
A Mushroom Retainer is a special adviser that assists the monarch of the Mushroom Kingdom in the country's administrative duties. Seven Toads that fill this role appear in Super Mario Bros., where they are kidnapped along with Princess Toadstool by Bowser, King of the Koopa. One Toad appears at the end of each castle and thanks Mario or Luigi for rescuing him. However, each one infamously states that their princess is in another castle. In Super Mario Bros. Special and Super Mario Bros.: The Lost Levels, these Toads keep the same role (in the latter, they also celebrate around the princess and her savior during the original ending). In the Super Mario All-Stars versions, an incremental number of them are seen being saved at a time from each of the first seven castles, depending on the current world.
The term has rarely been used since the original NES game, having only recurred in the video game series with the enhanced Game Boy Color port Super Mario Bros. Deluxe and in the digital manuals of Virtual Console re-releases. This is because Mushroom Retainer is intended to be an early localized name for the Toads as a species, along with Mushroom People. However, "retainer" implies that it is a castle job, thus it is distinct from civilians. The name and role that goes with it does make an appearance in other early media; for example, the Nintendo Comics System introductory story The Legend describes Toad as the royal Mushroom Retainer. The more reserved Wooster holds a similar position in the series, but (somewhat regretfully) directly serves the Mushroom King rather than the princess.
Names in other languages
In Mario and Luigi: Partners in Time when Peach is captured at the beginning Toadsworth says "You have to rescue her! And I assure you our princess is not in another castle!", a take off the infamous line.