Consonants and vowels all have different symbols in a dictionary guide in order to aid understanding. Some of the consonant symbols are fairly self-explanatory, such as "b" representing the sound of "b" in "bear," though the vowels and diphthongs are typically more complicated.
Consonant letters have such symbols as t? for the "ch" in chain, or d? for the "j" in jam. Other consonants, such as n simply stand for the "n" in "noun" and are much simpler. Nouns and diphthongs have symbols such as æ for the "a" in cat, rather than ? for the "a" in father. A comprehensive guide to dictionary pronunciation symbols can be found on Oxfordlearnersdictionaries.com.