Okay, so English language has a lot of words that are spelled the same, but they can have slightly different meanings depending on how you say them. So, people started adding little marks above or below the letters to show how to say the words correctly. These marks are called diacritical marks.
For example, the word "resume" can mean two different things depending on how you say it. If you say it like "reh-zoom", it means a document that shows your work experience. But if you say it like "ree-soo-may", it means to start again after taking a break.
So, to show which meaning you intend, you can add diacritical marks. The word "résumé" has a mark called an acute accent above the "e", which tells you to say it like "ree-soo-may". However, the word "resume" has no mark, so you say it like "reh-zoom".
Other diacritical marks in English include the umlaut (two dots) which are placed above a vowel to change its sound (e.g., "naïve" is pronounced "n-eye-eve"), and the circumflex (^) which is placed above a vowel to show length or stress (e.g., "crêpe" is pronounced "krape").
So, the diacritical marks just help us to say words correctly and avoid confusion!