Seriously, Tom, I have noticed that you get angry very easily since your examinations started.

seriously 是形容词 serious（严肃的、认真的）的副词形式，在这里用作语气词，表示说话者要强调接下来要说的话是认真的、真实的。放在句首时通常翻译为"说真的""老实说"。