The foundation wall of a 4 level split home has what our realtor called a frost line crack. It extends horizontally across about 5 ft. It is not large enough to fit my pinky finger into. There are also smaller cracks along the blocks diagonally toward the floor. It is the only wall with cracks (the front of the house). Does this sound serious? What exactly are frostline cracks and how do they occur?

Personally if I were looking at a home and the foundation had cracks larger than a hairline, or longer than a foot, I would walk away from it.

The house should be build to avoid frost heaving. There should not be "frostline cracks". You say it is not big enough for your pinky, but it must be close if you are using that as a desciption. This is an open crack is a very serious sign of shifting/settling. Walk away now or be prepared for some major foundation repairs.