A noise you make to mock someone — the sound of a child in a playground pulling a face and jeering, roughly "ha ha, you lost". It is British, it is old-fashioned, and grown people almost never shout it seriously today. Where you actually meet it is as a describing word for the way adults argue badly: "yah-boo politics", "yah-boo debate". That means shouting, point-scoring and name-calling instead of real reasons — politicians behaving like children in a playground. British newspapers and broadcasters use it in this way quite often, especially about noisy sessions of Parliament. The longer form is "yah-boo sucks", which is even more clearly the playground taunt. When you see it, the writer is nearly always disapproving: they are not joining in the jeering, they are complaining about it. So the word does two jobs at once — it makes the noise, and it judges the noise.