Like the pronouns qui, que, quoi, dont, and the forms of lequel, oł works as a relative to relate or join two ideas or clauses. Unlike qui, etc., however, où is not a pronoun, but strictly speaking an adverb. As a relative adverb, it can relate or join ideas or clauses in which the common element is an expression of time or place. For instance:

"Today is the holiday. I visit my parents on this holiday."
"Today is the holiday when (that) I visit my parents."

Or:

"This is the room. I work best in this room."
"This is the room where I work best/that I work best in."

The French equivalents would be:

"Aujourd'hui c'est le jour de fête. Je rends visite à mes parents ce jour de fête."
"Aujourd'hui, c'est le jour de fête où je rends visite à mes parents."

Or:

"C'est ici la pièce. Je travaille le mieux dans cette pièce-ci."
"C'est ici la pièce où je travaille le mieux."