The skin of an ability is the in-game justification for your character's possession of that ability.
For example, the ability to leap five paces is called 'Jump', which implies it is manifested through athletic prowess. An engineer may choose to apply for this ability as 'Spring-heeled Boots', and phys-rep having springs attached to their footwear. The effect remains the same; only the explanation changes.
A small number of abilities may not be re-skinned; these are clearly marked on the abilities list. You may skin an ability in any fashion you desire, but you should bear in mind that a clearly setting-inappropriate skin will attract a significant extra cost penalty (the "Magic Tax").