A swimbait is a baitfish imitation — most often a soft-plastic body with a paddle-shaped tail that flaps as you reel. Hard-bodied jointed swimbaits exist too (multi-segment hinged plastic) and run with more glide.
What separates a swimbait from a crankbait is profile and pace. Cranks dart and wobble erratically; swimbaits track straight with a steady, lifelike side-to-side kick. They shine when fish are keyed on actual baitfish — perch, shad, alewives — and the erratic action of a crank looks fake.
Rig a soft swimbait on a jighead for variable depth, or buy them pre-rigged with a weighted hook molded into the body. Slow, steady retrieve. Mix in occasional pauses to trigger a follow into a hit.