There could be any number of reasons for this; the first thing to do is to figure out what the liquid is. Antifreeze or water are the two most common issues; if it's antifreeze then that is commonly caused by a leaking heater hose or heater core. In the case of water it could be a bad door seal or a failed seal somewhere on the firewall.
Soak some of it up with a white paper towel and check to see if it has any color to it; if so see if it matches your antifreeze in the overflow container. Check for smell and see if it has a viscous consistency when you rub it between your fingers (wash your hands afterward).
Check all those things out then come on back here and let us know what you find.