A jig is a weighted hook ridden by a soft-plastic body (curl tail, paddle tail, twister), a hair skirt, or a feather. The weight at the head pitches the lure nose-first as it sinks, creating a wounded-baitfish profile that triggers strikes.
Cast and let it sink. Then either: lift-and-fall the rod tip to make the jig hop along the bottom, slow-roll it like a swimbait, or dead-stick it under a bobber for finicky fish. Standard sizes: 1/16 to 1/4 oz for trout and panfish; 1/4 to 1 oz for bass and walleye.