Giving War Subsidies is a diplomatic action that enables a nation to monetarily support another in a war. The recipient of the action will receive a certain amount of ducats per month which the giver decides.
The action can be assigned a certain amount of months from either 6 to 18, but it can be cancelled at any time for a small prestige and relations penalty. The diplomatic action is more likely to be used in a multiplayer lobby if the players support each other though money rather than though war. The AI may supply war subsidies if you are at war with one of their Rivals.