[MagnumOpus]

"Informed" in his context means based on new research/news/information, implying that the market is more likely than not to subsequently move in the direction of the trade.

A "toxic" position is one that has a high chance of moving against the holder. Taking the opposite side to informed trades might be toxic, but taking the opposite side to the first buy trade in a series of a hundred by a big mutual fund company who decided they like a stock is definitely toxic because they will continue moving the price higher with each trade -- so the brokerage doesn't want to keep these short positions on their own books but source them in the market.