Dont is a relative pronoun. It is "relative" because it relates or links two ideas or clauses. It is a pronoun because in relating or linking two ideas or clauses, it takes the place of the noun that is repeated in the second clause. For instance:

"There's the teacher." "I know that teacher."
"There's the teacher who(m) I know."

Or:

"There's the teacher. That teacher knows me."
"There's the teacher who knows me."

The French equivalent would be:

Voilà le professeur. Je connais ce professeur-là.
Voilà le professeur que je connais.

Or:

"Voilà le professeur. Ce professeur-là me connaîum;t.
"Voilà le professeur qui me connaîum;t."

    Unlike que or qui, however, dont replaces a noun that is preceded by de. For example:

"Voilà le professeur. Je parlais de ce professeur-là."
"Voilà le professeur dont je parlais.

"Voilà le professeur. Je connais la femme de ce professeur-là."
"Voilà le professeur dont je connais la femme."