It means that when you press on the gas, the power goes to the front wheels.
Normally, wheels keep your car rolling forward, not sidways., If you hit the gas hard enough that the wheels spin, which is easy to do in the snow, then the car can slide sideways.
Imagine a car driving in a circle. This is the same thing that happens in a turn, but it is easier to imagine in a circle. Imagine that the front wheels spin so the front wheels will slide sideways. This turns the car out from the circle, making a less tight circle. As you go into a less-tight circle, the tires get a chance to regain their grip. That's good.
Now, let's imagine your car is read wheel drive and going in a circle. You hit the gas too hard and the rear wheels start to slide sideways. Now, your car is turning into the circle, making it tighter, which makes the tires lose their grip more. And you end up spinning around.
There's always some guy who tells you that with proper driving skill, a rear wheel drive car is fine in the snow. Don't listen. A rear wheel drive car will be a problem in the winter.