A diacritic (/daɪəkrɪtik/) is a mark or glyph added to a letter, or basic glyph.
English does not have many words that contain diacritics. Exceptions are sometimes made for loanwords, particularly from French and Spanish; however, the diacritic is also sometimes omitted from such words.
They might be useful to distinguish words, e.g. resume from résumé, but nevertheless it is regularly spelled resume.