好好儿 is a spoken, reduplicated adverb with a retroflex suffix (儿化). It's used before verbs to add the sense of 'doing something properly, thoroughly, or with care'. You'll hear it in common phrases like 好好儿学习 'study hard', 好好儿休息 'rest well', and 好好儿说话 'speak nicely'. Unlike the adjective 好好 (hǎo hǎo 'good and proper'), 好好儿 has an intensified, casual tone and is typical of Northern Mandarin.