AgentWorkSkill in Salesforce Omni-Channel represents a skill used to route a work item to an agent. This object/entity is mainly used for reporting purpose. It contains the dropped skills also during routing. It is tied to the agent work object/entity.
AgentWorkSkill records are created after the AgentWork record creation.
Sample SOQL:
SELECT AgentWorkId, WasDropped,
Skill.DeveloperName, SkillLevel,
SkillPriority
FROM AgentWorkSkill
WHERE AgentWorkId IN (
SELECT Id
FROM AgentWork
WHERE WorkItemId = '5004W00002XZAcSQAX'
)