Other forms: yeahs
Yeah is a casual form of yes. When you’re hanging out with someone you really look up to and they ask you if you want to go to a party, play it cool by saying “Yeah, sure,” instead of “Yes! I’d love to!”
You would almost never write “yeah” unless you were trying to mimic the sound of talking. If someone says, "Yeah, right," they’re not casually telling you you’re right. They’re ironically saying “Not likely.” And if someone interrupts you while you’re talking and says “Yeah, yeah, yeah,” it means, “I already know that—–get on with it!.”