If behaviour is reprehensible, it is very wrong — wrong enough that people should say so, not smile it away. You use it of actions, remarks, treatment of others, more than of a person as a whole (though you can). It is a formal word. A friend might say “that was cruel”; a newspaper, a judge, or a speech is more likely to call a lie or an attack reprehensible. It does not mean “you can understand it” — that is comprehensible, which looks a little like this word and tricks the eye. It also does not mean a small bad thing, like forgetting homework. You reach for it when the blame should be public and serious. In ordinary talk many people would just say “that’s awful” or “that’s not OK.” You will meet reprehensible most in news, politics, and official criticism, not in the night market.