[1][2] Individual models differ slightly, but all consist of an axle stand on which the wheel can rotate and calipers, to measure slight deviations of the wheel's rim from ideal alignment.
Proper spoke tensions will help a wheel snap back to true if under specific load.
These truing stands include the ingenuity of many different kinds of materials to achieve the same quality as one found in a shop.
These materials include wood, old wheel forks, and even some with metal rails.
Although these are a cheap solution to keeping your wheel straight, they are not as precise as one from a well-known bike tool company.