Yep, pretty simple. A period is that wonderful dot at the end of a sentence that indicates the writer’s thought is complete.
It’s handy with simple sentences (noun & verb: John ran.), and (noun, verb, and anything else you can think of: Heart beating, muscles screaming, John raced down the path, away from the monster’s fiery breath).
It’s also handy with fragments of sentences–which should be used judiciously:
John walked down the pathway. He was almost to the end of the street when he felt it. The hot, fetid breath of the zombie-boyband clone.