Can someone please explain in simple English what one is and how I use it.
I am sorry if any of that was spelt wrong or my words are the wrong was around. English is not my first language.
Welcome to the forums.
The apostrophe is used to denote possession, "John's book" (the book that belongs to John). It is also used in contractions like "it's" (it is) and "he'd" (he had) amongst many others.