The Rest Bonus is a feature that is there for those people who cannot play all the time. For every second that your character is offline, a Rest Bonus Pool will increase in size by 20 Base Experience Points. Once you login again and kill a monster, for as long as you have XP in your Bonus Pool, you will get up to double Base Experience Points from that monster. The maximum amount of XP you can save up is about 1 week's worth. This Bonus Pool is specific to each character. Job Experience is unaffected by the Rest Bonus.
You log off in Prontera.
You log off for 3 hours. (3 Hours = 180 Minutes = 10,800 Seconds) According to the Resting System, you get 20 Base XP every second that they are offline.
That means 10,800 Seconds * 20 XP per Second = 216,000 XP accumulated total.
You log in at the end of the 3 hours and kill something that gives 15,000 Base XP. The XP will be doubled by depleting from the Bonus Pool.
You will then gain 30,000 Base XP for the kill and your Bonus Pool will decrease by 15,000 XP leaving you with 216,000 - 15,000 = 201,000 XP.
You will continue to get Bonus XP until your Bonus Pool is empty.
Note: Your Bonus Pool will only fill up for 7 days. If you log off for longer than seven days, your Bonus Pool will not continue accumulating beyond seven days (604,800 Seconds). That means your Bonus Pool can only accumulate 12,096,000 Base XP maximum.