We find that proprietary traders, be they fast or slow, provide liquidity with contrarian marketable orders, thus helping the market absorb shocks, even during a crisis, and they earn profits while doing so.A "marketable" order is one that "takes" liquidity. So what does this mean?
"Liquidity", even if we've narrowed the topic to "market liquidity", is not at all a single thing. An investor looking to buy or sell an asset for a few months, especially in a large quantity, can thereby push the market up or down to some degree; the proprietary traders to which they refer presumably are skilled at recognizing the signatures of those distortions, and will tend to enter the market on the other side, such that over the course of a day the net purchase of the proprietary traders and the longer-term investor combined is closer to zero. The proprietary traders may well exit their positions a week later; they are specialized in more granular timing than the longer-term traders are. They, however, enter and exit their positions by trading directly with even shorter-term traders, who will typically close their own positions the same day that they take them, and often the same hour. Many sorts of traders, thus, effectively provide liquidity to those trading on larger time scales than themselves, taking it from those trading on shorter time scales than themselves.