The table shows the requirements and amount of EXP gained for completing a chapter in normal and hard mode.
The number of turns indicates the number of turns you must beat the chapter to get the full bonus. If you take longer the amount of EXP you earn will decrease.
You will also earn EXP when a green or yellow unit defeats a red enemy unit. You can use Tanith's skill to summon 2 Pegasus knights and 1 Falcoknight (total of 6 ally units), any EXP earned by these units will go to your Bonus EXP.
(Thanks to Rick for the tip).