RPG Maker allows your actors to level up by reaching the amount of experience points required. The amount of exp required is determined by the actor's level for their current class, and the class' exp curve.
Exp curves are generated using a pre-determined formula, and you only have the ability to adjust different parameters. In some cases, your desired exp curve is simply impossible to achieve with RPG Maker's own exp curves.
With this plugin, you have full control over exp values requires for each level. For example, if you wanted to make it so that every level required 1000 EXP, you could simply set the exp required at intervals of 1000, without having to figure out how to adjust the curve.
You can manage your exp tables in a spreadsheet as well, allowing you to take advantage of spreadsheet software to manage your data.
More information and downloads are available at HimeWorks