Skills are created in the system, and users attribute these to their profile with a proficiency rating, which can be viewed by others in the 'expertise' field.
An extra optional function on top of this is the ability to allow others to endorse users for Skills.
This is facilitated using the hardcoded 'Endorsements' field, and this guide details how this works.
1. Make sure the 'Endorsements' field appears
An application administrator of People can set the 'endorsement' field to appear on user profiles so it can be used in Admin > People > Configure profile fields > 'View profile page':

Once saved, the field will appear on profiles on the front end of People:

2. Users can endorse others
A user cannot endorse themselves for Skills; this can only be given by other users.
When browsing user profiles in People, endorsements can be made using the corresponding field.
The list of Skills that appears here is the same as configured by administrators on the admin side or those with 'create skills' permission.
Single or multiple skills can be endorsed for a user at a time:

Once added, a blue cross appears, which allows other users to begin endorsing the same skills when viewing the users profile.
The total next to each represents the number of endorsements for that Skill.

Users will receive a notification for each endorsement made for them.
Endorsements are limited to once per user per skill.
Endorsements can be removed by clicking the blue cross again:

3. Endorsement search
The Skills search can be accessed via the eye icon given on the front end of People or when on a user's profile.

Users can apply the 'endorsements' option to return users and the total number of endorsements for that skill.

4. Expertise vs Endorsements
Both of these fields & functions are optional to use.
Both of these fields use the list of Skills created by an administrator and those with the extra permission to do so, as set on the admin side of People (Admin > People > Configure skills)
However, the 'Expertise' field lists skills users attributed to themselves in the 'edit my profile' area alongside a self-evaluated proficiency rating.
In contrast, endorsements are skills other users have attributed to their profile and can be further validated by other users endorsing them.
Both can be searched in the skills search, but the intention behind them is different, so your team can choose to implement one or both fields to fit your use case and how you want your users to interact with Skills.