Full tang

In a full tang knife the handle scales are attached to either side of a continuous piece of steel. That makes the knife stronger, better balanced and much less likely to fail at the point where blade meets handle, which is where cheap knives break.

The alternative, a rat-tail or partial tang, is a narrow spike of steel epoxied or pinned into a hollow handle. It is lighter and cheaper, and it works fine until you lever against a joint.

On a fillet knife the balance point matters as much as the strength. A full tang puts weight back toward your hand, which makes long cuts steadier.

The one argument for a partial tang on a fishing knife is corrosion. A full tang has an exposed strip of steel running between the handle scales, and on a knife that lives wet that seam is where rust starts. Sealed or fully encapsulated handles get around it.