Cogset - axle spacing

fork - dropouts

rear fork - rear triangle

You bought wheels with an OLD (over locknut dimension, also known as spacing) that is less than the distance between the inside of the dropouts. You need to add spacers (NOT washers) until the locknuts fit snugly within the dropouts. Then you can determine on which side of the hub those spacers need to go to get the chain nice and straight between the chainring and your cog. Finally, you will probably need to have the wheel dished (rim moved laterally relative to the centerline of the hub) in order to get it centered in the dropouts.

Road bike frames most commonly have a rear spacing of 120mm (track/singlespeed), 126mm (vintage road), 130mm (modern road), or 135mm (trekking/touring). Your axle must have an OLD to match.

Did this rear wheel have multiple gears on it? What frame are you putting it in?