Every Capital City within Black Desert can be ruled over by a Guild, and thereby the leader of said Guild is established as Lord of the city. This allows the collection of taxes and setting up percentage of the taxes, as well as a few additional benefits covered in the Siege portion of the wiki.
City Information Menu
Interacting with the lord of a Capital City will provide you with a window that lays out in detail the current taxes, level of unrest, and the guild that currently owns the city.
Current Taxes explains the percentage of monetary gains being deducted from transactions within the city and given to the guild bank of the ruling party. (Taxes can be set from 0 to a maximum of 10 percent)
The level of unrest allows you to see the status of the citizens within the city and their relative happiness. A low happiness directly impacts the maximum amount of revenue earned by the ruling party, and also spreads in a positive or negative manner dependent on the actions taken by said party.
The ruling party of the city is essentially the last man standing at the siege of the city, upon successfully wining the siege the previous ruler is kicked out.