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 $500,000. For each day that you consecutively login, you add $500,000 to that amount, up to a maximum of $2,000,000 per day. When you miss a day, you go down two steps, not down to zero. If you consistently log in every day, your nation can earn up to $14,000,000 per week just by being active.
New players with nations less than 60 days old have a maximum cap of $4,000,000. They have the same $500,000 minimum, and increment by $1,000,000 per day.