As your city and army develop your general will gain more experience and will obtain skills that empower your army and city even more. Whenever your general reaches a new level you will be awarded Skill Points that can be spend to improve Skills and unlock new ones. There are two Skill Categories - War and Development. Each category consists of Individual Skills and Skill Clusters.
Each cluster consists of a group of skills and allows you to progress further in the category after a specific amount of Skill points are spend on it.
Note that the " /15" Points in the center of the Cluster show the maximum amount of Skill points that you can spend on the Skills in that cluster. Once you complete a cluster you will not be able to spend more Skill Points on the Skills in it.
You can distribute Skill Points in Clusters in any way you like.
If you change mind about a Skill Point or two you can always use The Skill Reset item and to set them just the way you want.