Could be wheel balance, loose wheel bearings, bad drums/rotors, loose tie rod ends, worn steering rack, cords separating on tires. In my case, it was Old Struts. Like Old Crow, only you can't drink it..
It could mean MANY things.
The only thing it ALWAYS means is vibration from SOMEWHERE is being transmitted up the steering column.
The source of the vibration can be the road, the tires, the suspension, or even the engine.
The best place to START is having the tire balance and alignment checked.
Note: While misalignment won't cause vibration, checking alignment will likely spot suspension issues that may cause vibration.