PLS is please, written short. You meet it in text messages, chat, and quick email, where people are in a hurry and do not type the whole word. They also write pls in small letters, and some write plz. It is only for writing, and only when the talk is casual. You would not put it in a school essay, a letter to a teacher, or any careful email. All-caps PLS can feel a bit loud, like a push; small pls is softer. Older people often still write please in full; on a phone, younger people almost always shorten it. It does not change the meaning of please — it only changes how the word looks on the screen.