If the water only shows up when you run the AC it is, as Jeremy said, it is the AC condensate drain…and it is easy fix for either you or your mechanic.
If the water shows up anytime or when you run your heater and smells “bad” it is a leak in the heater core. This is NOT an easy fix on your car and will be several hundred bucks.
If the water shows up after a rain it is PROBABLY a plugged cowl drain. These are located 1 on each side of the cowl (the slotted thing just in front of the windshield) and can get plugged with leaves and dirt. You may be able to stretch out a coathanger and poke it around to clear it, but the only “permanent” way is to remove the cowl and then clean out the leaves and dirt. This is sort-of a medium hard fix that you might be able to do yourself. Sometimes a mechanic can blow compressed air up from the bottom of the drain (if they can find it!) for a temporary fix, but eventually the leaves will wash back.