Similar to other Bigpoint games, the Daily Login Bonus is a reward that you can get by logging into your game every day. The longer you log into the game, the higher the rewards that you can collect.
Currently the rewards will count on towards 5 days to get a special prize. In order to get to the fifth login day you must collect your bonus every single day for 5 days in a row.
If you reach the fifth day for the first time, a "Special reward" will await you.
* Where can I find the Daily Login Bonus?