The skill history will give you an overview of the differences between the current and the past average skill levels. Based on the skill grade in employee´s profiles in your organization, you will be able to see what the skills grade was in the past and what skills lever they have today and compare this over a period of time.
Go to Reports -> Competences and the tab Skills history,
to the right, you can view the average skill level and users of the skills in this case "Last year" (See the filter) and
to the left, you can view the average skill level and users of today.
In this image; ".Net" has an average level of 3 last year(2021 - 04- 04) and the average level 3,5 today.
Use the filter to set a From date. You can also filter your Skill sets to get an easy overview of the skill history related to the skill set. It is also optional to filter on teams and tags.
At the bottom, you will view the average level of all skills in your company or your skill sets depending on the filtering.