lgb: "Index" is a sorta indication of overall market direction, if Index is down it shows more shares were traded at lower price than yesterday's value at COB. As for your million or should I say "billion" dollar question "how do people know the flow?", nobody does. Some people are good at guessing but no one knows for "sure" when to get out of a stock.
How do people get into this line of work? Do you mean how to start trading shares? If thats the case then you simply pick one brokerage firm and open an account with them and start your trading.
"Volume traded" means how many shares were "traded", or how many shares/stocks changed hands.
shadow? I have not seen any brokerage firm offer their services for “free” (at least in US), there are cheaper ones though but I’m not so sure about their quality of service.