I'm sort of confused on the investing term "Yield". I know that it represents an annual income based on a certain percentage of a stock invesment, but am a bit unsure, how it is used? and what is the advantage/disadvantage of a negative/positive yield?

Also I noticied when a stock is at an all time high it's yield usually lowers quite significently. Can someone please tell me what is the cause of this? or is it just overinflation of the stock?