A simple google search could have answered your question:
Kingdom of Miscellania Management 
To be able to start managing your kingdom, you must first complete the quest "Throne of Miscellania". I highly recommend you to do "Royal Trouble" after the first one. You will have extra workers, and more optional resources among other things.
So, what is this kingdom managing thing?
Basically you're a manager of a team. Your team is made out of workers who gather resources, from trees, ores, tropical wood, to seeds, flax, etc. You must command them to gather the resources by talking to advisor Ghrim after "Throne of Miscellania". And as days pass, they should gather the resources you asked them to gather. Of course, they don't work for free...
This is where the coffer comes in.
RsWikia wrote:
Every day at midnight UTC, the citizens deduct 10% of the balance (but only up to a maximum of 50,000 or 75,000 coins, depending on which quests are completed) and convert it into resources. The rewards can be extremely good if players have both a high approval rating and high amounts of cash in the treasury. For example, with just over 500k coins invested (50k coins deducted from treasury), each day players could expect to receive 370 coal and 20 herbs by setting mining to maximum and herbs to as high as it will go. All herbs are tarromin or better, making this option a good return on investment.
Players need 500,000 coins (750,000 coins after Royal Trouble) in the coffers to get the maximum deduction for one day. To go seven days without collecting resources, players would need 850k coins (1.275M after Royal Trouble) in the coffers to get the maximum amount of resources. If the player deposits the maximum of 5,000,000 or 7,500,000 coins, he/she could go up to 90 days without collecting resources and still get the maximum deduction for one day (however, the player still needs to go back to the kingdom periodically to maintain a high approval rating if he/she wants to receive a decent amount of resources).
If a player has completed only Throne of Miscellania, it is sometimes recommended not to deposit into the coffers until both quests are complete. Waiting until both quests are complete limits possible ill effects of not topping off approval when approval drops 3% per day.
Myself, I always have 2M+ there.
In the end, with the right combinations you'll be able to get a rather high profit from this.
Topic: "Best Combo for Kingdom Management?"
