Frontal Inversions are inversions caused by a shallow "cold front" blowing in under warmer air.  In other words, sometimes a bunch of cold air, called an air mass, will get blown by the wind from one place to another, warmer place, and will get blown underneath the warm air, causing an inversion.

Sometimes you hear about a "cold front" or a "warm front" moving in from somewhere.  What this is referring to is when wind is blowing from a cold or warm place, and causing a large pocket of that air, also called an "air mass," to come to where you are.

If this is a cold air mass, and it is small enough it blows under the current, warmer air without pushing all the warm air away, this can cause an inversion