A diacritic is a little mark you add to a letter — above it, below it, or through it — to show that the sound is not the same as the plain letter. The accent on café, the mark on naïve, the wavy line on Spanish ñ, the two dots on German ü: those are diacritics. English itself almost never needs them on home-grown words; we borrowed the marks with foreign words, and even then many English writers just drop them and write cafe. In other languages the mark is not decoration — it can change the word. Though actually, if you have learned Pinyin, you already know this idea from tone marks (mā, má, mǎ, mà), which are diacritics too. Teachers also say accent in a loose way; that can mean the mark, or the way a person speaks. Diacritic is the tidy word for the mark.