The daily login bonus is a reward for being an active player and checking on your nation every day.
Every day that you login, your nation earns a minimum of $100,000. For each day that you consecutively login, you add $20,000 to that amount, up to a maximum of $300,000 per day. If you consistently login every day, your nation can earn up to $2,100,000 per week just by being active.
New players with nations less than 60 days old have a maximum cap of $600,000. They have the same $100,000 minimum, and increment by $20,000 per day, but can earn far more money over their first 60 days by logging in once per day. New nations can earn $600,000 daily by their 26th day of consecutive activity, and can earn $29,500,000 over their first 60 days just by logging in each day.