Diplomacy summarize all the different gameplay mechanics for interactions between States (players).
The main diplomatic actions involves creating and sending diplomatic Treaties to neighbors whose terms become active if they agree to it. These agreements allow to exchange ideas and knowledges, form Nations or to expand your trading network,
Other more specific diplomatic actions can be to designate a rival, or to fabricate a territorial claim to gain a casus-belli against another State in order to invade a territory.