Bukkit

The new values provide a way to control the outcomes of enchantments in the table from what levels can be obtained, when they can be obtained, and how easily they can be obtained.

The usage for these depends on the modified enchantment level, which is a random number based off of the experience cost selected and the item you are enchanting. The item affects it by what type it is (such as diamond, gold, stone, etc.). Here are some values for possible modified levels:

Diamond:
Exp Level 1 results in a modified level between 2 and 6
Exp Level 30 results in a modified level between 26 and 39

Gold:
Exp Level 1 results in a modified level between 2 and 15
Exp level 30 results in a modified level between 26 and 48

Half way between the ranges (such as 36 for gold or 32 for diamond) have a much higher chance than at the extremes of the ranges. For example, getting a modified level of 36 for a level 30 gold enchantment has around an 11% chance of happening while getting the maximum 48 has around a 0.01% of happening. Further tables can be found here

With the ranges in mind, you can use the base and interval values to set up what levels can receive enchantment levels. The level is determined by the following formula:

enchantment level = (modified level - base) / interval + 1

That formula rounds down and is limited by the maximum level so it cannot go above. If the level is less than 1, the enchantment is not applied and a different one is chosen. You can use this to help determine what values will suit what you want to happen. You can also use the default vanilla values as reference if you want to have similar effects.