What is a tilde?

A 'tilde' is the '~' mark placed over the letter 'n' as in the Spanish word 'Señor'.

ex: ¿Cuántos años tienes? --> How old are you?

In Spanish, 'ñ' is a letter of its own just like 'a', 'b','c'. 'año' = 'year' and 'ano' = 'backside (ass)'. Therefore, it is important to distinguish between the letter ñ and the letter n.
A tilde is a character on your keyboard. It looks like this: ~ . It is not used in the English language, but some Latin languages use it as a letter modifier. It is pronounced 'TILL-DUH'.