(last updated: 12/28/17)
Learning how to take a punch is the first thing most people learn on the road to being tough. Soak represents a fighter's capacity for taking physical punishment--it can represent her iron physique, or his iron will. Either way, a character with a lot of Soak isn't going to take a beating lying down. Streetwise toughs with a lot of miles logged in the alleyways tend to have a lot of Soak, as do large bruisers and master monk types.
While Soak does tend to go hand in hand with a relatively good Physical competency, this is not always the case. A more pacifist person might be used to taking hits, but hasn't been formally trained in fighting back. Likewise, a fighter with high Aura or Chi might have some sort of secondary technique for reinforcing their body against assault. Soak might also represent the presence of some sort of full body armor that protects the wearer from assault at all angles. It's worth noting that a shield, while being able to block blows like armor might, does not itself justify a higher Soak in a fighter; a shield is an active defense, and as such informs an ability of some kind, such as Brace or Parry.
Still, with high enough levels of Soak, a master of the arts can become nigh-impervious to physical damage, being able to do things like block sword strikes or bullets with their bare skin. Characters with high Soak might even have some sort of limited internal regeneration or protected organs, able to recover quickly from critical blows and injuries that would kill normal fighters on the spot.