The true-ness of a wheel is how near to circular it is.
It should have a constant radius and the sides of the rim should be the same distance from the centre plane along the whole circumference.
Wheels are held in shape by spoke tension, but often, factory wheels don't hold their tension, or become damaged. Professionally handbuilt wheels are much stronger and hold their shape better.
You build and maintain the wheel using a spoke key. it is the cheapest bike tool, but capable of inflicting expensive damage. You have to use it with caution, no more then 1/8 of a turn at a time, when correcting an out-of-true wheel.
There are some good books and readings, the best being The Bycicle Wheel by Jobst Brandt.