It's supposed to mean that the API responds with JSON data (as opposed to, say, XML). "REST" here just means we use standard URL patterns and rely on HTTP verbs (e.g. GET), to get stuff done.

I think you're missing a "currency" field in the request, particularly since you support both AUD and USD.

I wouldn't call it buzz phrase, I see JSON REST API and I know exactly what it is and what to expect from it.