在乎 (zàihu) means 'to care about, to mind' and is used for things that matter to you emotionally or personally, often in negative statements. It's less formal than 介意 (jièyì 'to take offense'), which implies being bothered or offended, and more about personal concern than 关心 (guānxīn 'to show care/concern for someone'), which involves active attention or worry.