Silent letters are letters written in words that do not correspond to pronunciation of a given sound and do not even allow people to hear any sound in the word. Many of them come from Old English, when the sound had been heard years ago in the past. In English, these include the k in know and knee, the second b in bomb and numb, the s in island, and the e in lame and lime. There is also a silent g in sign, which is from Middle English. You can sometimes make a switch from damn to damnation, because of the n sound. Old English uses the sound of the silent k and gh from knight, which was pronounced at the time. The word changed its history. Other silent letters may be the r in words from Old French, like hors d'ouvert and the sound of the t is sometimes heard in some words like often, but it is also silent as well as that. There is also a silent w in answer, and the w is recognized to keep quiet. Pronouncing the w is prohibited. The word kneel starts with a silent k as well, which is instead of being followed by an r, it is followed by an n and words that end with silent n are different because one of them is autumn. Also, there are words that end with a silent e because another vowel is saying its name.
Silent letters exist in the spelling systems in many languages, but they are very common in languages with deep orthographies, spelling systems with little one-to-one relationship between letters and sounds, like English, French, Thai, and Mongolian.