When you do something impeccably, you do it so well that nobody can fairly pick a hole in it. It is the adverb of impeccable: without fault, and hard to criticize. People reach for it about clothes, manners, taste, and timing — someone may be impeccably dressed, impeccably polite, or have impeccably good timing. It is rather formal and a little elegant; you would not usually say a sandwich was impeccably made unless you were being grand on purpose. Though actually, speakers often stretch it to mean only “very, very well,” not “literally without a single fault.” In careful use it still carries that “you could not criticize this” feeling. It is strong praise.