[Don't worry, what he said] is a joke.
joudan: joke
joudan + "da" declares that e.g. something that was said before, is a joke (in the sense "just kidding"). This is a fully valid statement, which reveals an important detail about Japanese. In English you'd say: "[This] is a joke". Japanese also has words like "this" or "that", but they are usually omitted - especially when the context these words refer to is known.
Throughout this course, I will denote missing context by square brackets. So anything in [these brackets] is clear from context and sometimes required to understand the expression.
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