These skill sets indicate your character's Abilities in a particular field of skills.
If you want to perform an action you never learned explicitly, you must attempt a roll on the value of the skill set to which the corresponding action belongs.
What do these skill sets mean?
A physical action is what your character actively does, usually with physical consequences. It doesn't matter if you want to cut down trees or ride a horse, all these skills are checked against Physical.
Your character needs knowledge, for example, to understand a foreign language, to be able to classify political connections, to understand mathematical principles or to be able to differentiate plants and animals.
Social skills are particularly necessary when interacting with NPCs. For example if your character wants to haggle with a merchant, try to manipulate someone’s thinking or see if they are being lied to based on their knowledge of human nature.
To look up
The values of skill sets are calculated by dividing the sum of the skill points used in a skill set by 10. It is rounded up commercially (i.e. from 5 upwards). Example: 123 points spent in physical = 12 points in physical 129 points spent in physical = 13 points in physical