A is a word that takes the place of (or represents) a noun. It can do anything a noun can do. For example, it can be the subject of a sentence (or the direct object, the indirect object, or the object of a preposition).
Examples: “he”, “she”, “me”, “those”

ἐν αὐτῷ ζωὴ ἦν John 1:3

en autō zōē ēn

in him life it was

In him was life

** αὐτῷ** is a taking the place of the noun “λόγος”.