A diacritical mark is a symbol that tells a reader how to pronounce a letter. Part of learning to speak and write a new language is becoming familiar with its diacritical marks.
Diacritical marks can be squiggles, lines, or dots, and they can hover above a letter or be attached to it. They're also known as diacritics or accents. No matter what you call them or what they look like, diacritical marks are there to show you how a letter sounds when you say it out loud. The word café, for example, includes a diacritical mark that tells you to pronounce that last e as "ay."