Peace is a mechanic in Hexagor.io. It is used so that two Empire's may not attack each other in any way. Making peace with someone can be very useful, and is best used making peace with all opposing large Empires near the player's Capital, so that there is a barrier to protect you from other Empires, as well as those ones.
To make peace with another player, go to the Alliances Menu at the top-right, and click send request, then click on any other Empire's territory. Both Empires must agree to make peace for a peace treaty to work. When receiving a peace request, you can select the check mark at the top-right of the game.