Usually if it's said out loud at the end of a sentence it means that the speaker is trying to audibly mark the end of something, or emphasizing a point. For instance it could be the end of a conversation, argument, or a declaration.

Period simply refers to the dot (.) put at the end of a sentence. If it's pronounced the speaker usually intends to emphasize that the sentence is over or that enough has been said.

A person may be dictating English to someone who is writing it down. If he wants to be absolutely precise and make sure that the transcription is completely accurate, he may speak aloud the names for the punctuation marks. More commonly, it is used to mean "this argument is over, it's finished, I'm not going to discuss it any farther." For example: "Can I play a video game on your iPad, Grampy?" "No, you've had enough screen time today." "Not even if I'm very good?" "No, not even if I'm very good." "Not even if I promise to go right to bed afterwards?" "No, not even if you go right to bed afterwards." "Not even if it's an educational game?" "No, no, no. No games on the iPad, period. Go brush your teeth and get into your pajamas."

"Period" это "точка" в американском-английском языке. В британском-английском языке "точка" називается "full stop". Когда Американцы говорят "period" в конце предложенния, это значит "точка!", "все!", "разговор закончился!" Или они диктуют.