Callan stage 3 lesson 25 at page 148 starts with "What do the letters A.M mean after the time?"

レッスンで出てきた文法・単語	vocabulary - A.M , P.M, Latin, midday, sometimes - a lot of, both .. and - a lot - If, must, either..or, bar, restaurant ============================== grammar - Instead of "much" and "many", we can use "a lot of" both for things we can count and for things we cannot count. Instead of saying "many books", we can say "a lot of books". Instead of saying "much water", we can say "a lot of water".
レッスンで利用した教材	pdf file Callan stage 3
レッスンで利用した教材のページ数	148-150

A.M = ante meridiem
P.M = post meridiem

The letters A.M mean ante meridiem after time.
The difference between A.M and P.M is that we use A.M before midday, whilst we use P.M after midday.