A pronoun is a word that stands in for a noun. The word goes back to Latin (pronomen), and literally means in the place of (pro) a name/noun (nomen). That’s why pronouns usually come after the noun that they refer back to:
John‘s robot even combs his hair for him.
Here the pronoun his refers back to John, a proper noun. Bonus points if you noticed that the same applies to him.
The noun that comes before the pronoun also has a fancy Latin name: it’s called the antecedent (from Latin antecedere, to go before).
It can happen that the pronoun comes before the noun, as in this example:
In his last will and testament, Carl left his wife the whole kit and caboodle.
It can also happen that a pronoun has no antecedent, as Lewis Carroll famously pointed out in Alice’s Adventures in Wonderland. In this passage, the Mouse is trying to read a history lesson, when the Duck rudely interrupts him:
[Said the mouse,] “…even Stigand, the patriotic archbishop of Canterbury, found it advisable—”
“Found what?” said the Duck.
“Found it,” the Mouse replied rather crossly: “of course you know what “it” means.”
“I know what ‘it’ means well enough, when I find a thing,” said the Duck: “it’s generally a frog or a worm. The question is, what did the archbishop find?”
Whereas the Duck insists that language should follow the rules, and that it must refer to something (the institutional view of language), the Mouse simply ignores his objection and takes the view that his intended meaning is clear (the intentional view of language).
As the Duchess will later say, “take care of the sense, and the sounds will take care of themselves.” If life were only that easy.