You are a cypher. Hidden from the government and it officials trying to make enough New Yen to get by.
The world contains the those with SIN and the SIN less. If you have a SIN then you can be traced and watched by those who employ you. But without a SIN you don't legally exist. You become a Shadow, running missions to earn New Yen to keep a roof over your head.
This is a futuristic game where the players take on missions from various organisations, Running through the Shadow places. Hence the title. The world is dominated by huge corporations which rule the planet. All people are given IDs at birth, Standard(?) Identification Number (SIN.) But there are a few which manage to "escape" this and live without the direct involvement of the government or the business suits. But you gotta eat (and drink and take weird substances.) So the sinless (those without SIN) must do the bidding of someone.
Enter Mr Johnson, a man with a problem, that the traditional corporate world cannot fix within the mainstream culture. This gives the players an entry point. They can get away with murder (sometimes) and resolve these pesky little things.
A typical venture would be to steal a valuable prototype for a rival corporation. If you are caught no blame can be traced back to anyone important. I mean, you did destroy those data chips, didn't you?