This is the 11452nd most frequent Russian word.
договору
"Договору" is the dative case of "договор," meaning "agreement" or "contract."
The word 'договору' is used in the dative case to indicate the topic of agreement.
Мы пришли к соглашению по договору.
We have reached an agreement on the contract.
The word 'договору' in the dative case specifies the entity whose terms were altered.
Условия по договору были изменены.
The terms of the contract have been changed.
Here, 'договором' uses the instrumental case to describe the means through which something is stipulated.