Well you can say
John is playing. "John" is a noun.
He is playing "He" is a pronoun
A pronoun can be used in place of nouns.
Pronouns are words like he,she, it, him, her, ..
I just found this.
It is the same for numerals and pronumerals. Pronumerals (letters) can be used in place of a number.
for example in the maths expression 5y y is a pronumeral.