Could someone explain to me the meanings and use cases of "já já"?
It really depends on the context. For an example, if you are waiting for your dish and the waitress says "vem já já", it's like a friendly way of saying that it will be here soon, but that she doesn't know the exact time. Another example: "Ele está chegando já já" means that he is arriving soon, at any moment. The word já used by itself can be used in a more formal way, like in an order: "vem cá já!", said to a kid, means that he should get going RIGHT NOW :)
Já - dá a ideia de imediatamente, agora ou "neste instante".
Exemplo: Já vou!
Já, já - dá a ideia de "daqui a pouco" ou "aguarde só um momento"...
Exemplo: Espera que já já eu vou.
Já - it gives the idea immediately , at the moment , right now.
Já, já - it gives the idea of "a little later" or "wait just a moment".