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In simple_action_client.h it's defined like this:

void sendGoal(const Goal& goal,
                SimpleDoneCallback     done_cb     = SimpleDoneCallback(),
                SimpleActiveCallback   active_cb   = SimpleActiveCallback(),
                SimpleFeedbackCallback feedback_cb = SimpleFeedbackCallback());

In C++ you can specify default function argument value with = sign. It's an optional value and therefore doesn't need to be present while calling that function. If you don't specify one it will take the default value (e.g. SimpleDoneCallback()).