What characteristic allows liquid water to flow on present-day Mars, despite the cold environment?

Highly concentrated salt solutions, or brines, which have a lower freezing point

Although Mars is currently very cold, liquid water is observed today through transient flows known as Recurring Slope Lineae (RSL). The persistence of this liquid state is made possible because the water is not pure. It exists as highly concentrated salt solutions, referred to as brines. These salts depress the freezing point of water significantly below the ambient Martian temperature, allowing the water to remain in a liquid phase long enough to flow seasonally or cyclically down slopes. While the volumes are small and the flows rapidly evaporate or soak into the ground, this confirms that conditions conducive to liquid states still exist in localized, specific areas.