Imagine you're in a space ship in the very early Solar System, before planets form from the protoplanetary disk shown here.  At the frost line, about where the asteroid belt will be, it gets cold enough for ice grains.  When you pass this line, the density of solid particles in the disk abruptly increases by a factor of 3 or 4.   So, these particles will stick together to form larger bodies - and faster, too!  This means that gas giants are more likely to appear beyond the frost line, since the bodies that form beyond this line are bigger and have more time to accrete gas from the disk before it dissipates. 

The frost line is also called the snow line, and you can read more about it here:

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