It is meter which can effect the relation between civs.

In early era the meter will effect regions and as the world will get globalize the effect will be globally.

It can be effected by trade, peace deals, declaration of war, etc.

The meter may fluctuate up and down

If it increase there will be more chances of great war or invasion. There can be more alliance formation and intrusion of military in border or skirmishes by aggressive civs or rise in aggression of enemy civs.

If it falls there can be less military alliance and less chance of war or skirmishes.