While it may be possible to true a wheel through tightening spokes if they are loose or incorrectly tensioned. It is not possible to completely straighten a buckled wheel.
A straight wheel requires a perfectly circular rim, the spokes serve the purpose of both positioning the rim relative to the wheel hub and providing strength to the wheel through tension. If a wheel has been buckled due to hitting a curb or other damage, the rim is no longer circular. At this point tightening the spokes may no longer work, and rather than straightening the rim, a flat spot on the top of the rim is created through excessive tightening.
It is important to regularly check the wheels for damage and check spoke tension. If a wheel sustains a serious buckle it will need to be completely replaced.