The number in brackets indicates the level of the skill, which for the most part corresponds to your own skills in terms of damage and percentage chance to hit, etc. However, Frost on a weapon will ignore the target's resistances, and so will work on creatures that Freeze does not.
For detailed descriptions of skills in your own skill set;
The Locus Inn - Skill System (individual paths, and a skill table for each class)
RPG Vault - Divine Divinity Skills XI (links to other skill sets at bottom of page)
