Through hardened means that the entire blade is tempered homogeneously -- that the most common way that knives are manufactured. However, the best cutting performance is often achieved by hardening the edge to a degree that, if applied to the entire blade, would make it brittle. One approach to deal with this is laminated steel; a very hard layer of steel is sandwiched between two softer layers. The inner layer provides a very hard edge, while the outer layers keep the blade from breaking under stress. Another approach is to differentially tempered blades (as you see in traditional katanas). The steel along the edge is harder than the steel closer to the spine. The area of transition between hard and softer steel has a different appearance, called a hamon line. Cheap monosteel blades sometimes use an acid etch to simulate a hamon.

My understanding from reading about different Swamp Rat releases is that the through hardened models are cheaper to produce and hold up just as well as the differentially tempered ones, so I don't know if this blade will be differentially hardened or not.