Every other leader in the game has an opinion of you. Their opinion might be positive or it might be negative; it is in constant flux from events in the game. You can tell what your relationship with another leader is by the icon on their leader portrait, or by selecting Our Relationship with that leader on the Diplomacy Panel. The following relationship states exist in the game:

Allies. Alliances have their own section in the Civilopedia where they are described in detail. These can only be created between Declared Friends, and provide Open Borders and a Defensive Pact between the partners in addition to Alliance's benefits. Civilizations in an Alliance cannot declare war on each other. You must have the Civil Service Civic to create an Alliance.

Declared Friend. When you choose to declare your friendship with a leader, you may not declare a formal war on each other, and deals you make with each other will be more favorable. This relationship state is required to make an Alliance.

Friendly. This means the leader thinks positively of you and you have a better chance of getting favorable deals with them.

Neutral. You have done nothing to upset or impress the leader, and your interactions with them will be standard.

Unfriendly. For some reason, you have done something to upset this leader, and deals with them will likely be unfavorable for you, or you can choose to make demands of them.

Denounced. This leader has let the other leaders in the world know that you have done something to offend them. Your deals with them will be unfavorable if they will make them at all, but you can declare a Formal War against them and make demands.

At War. Open hostilities have broken out between you, and you can attack or invade this leader but you will accrue warmongering and war weariness penalties.