mase, (a form of conjugation).
Walking on the street, various different types of expressions are heard --- those who are living here naturally became to ignore words without special meanings, but those who are visitors tend to pick up much more. Here is a such example: a student walking besides me asked what does it mean with ませ in いらっしゃいませ. As we have enough time to expand a discussion, so I brought up the topic of verb conjugations and how it works in the word ます.
It is always a sensitive judgement as how much grammatical discussion is necessary for a language learner. However, I am a believer that it is not only useful but essential for an university student to always realise language rules behind expressions.