You say 'Yay!' when something good happens and you feel happy right then — a friend passes an exam, the rain stops, the food arrives. It is a small shout of joy, not a word with a meaning you can put in another sentence; it stands alone, usually with an exclamation mark. It is warm and a bit childish, which is exactly why adults use it: it makes you sound light and friendly instead of serious. Very common in texting and online chat, where people also stretch it — 'yaaay' — or add it after news to show how they feel: 'She said yes, yay!' Because it sounds young, be careful at work; 'yay' in a message to your boss is fine between friendly colleagues but odd in a formal report. There is also a sarcastic use, said flat and slow: 'Oh. Yay. More homework.' — meaning the opposite, and English speakers hear the difference from the tone. One more use you may meet: people hold two fingers apart and say 'yay big' or 'yay high' to mean 'about this big', showing the size with their hand.