Carefully and thoroughly; taking one's time — an adverb meaning to do something without haste and with attention to detail.
問題をじっくり考えてから答えてください。
Please think about the question carefully before answering.
この料理はじっくり煮込むと味が良くなる。
This dish gets better if you simmer it slowly and thoroughly.
時間があるときに本をじっくり読むのが好きだ。
I like to read books carefully when I have time.