What does re-tensioning spokes - truing a wheel mean?

Re-tensioning spokes: Re-tensioning the individual spokes on a wheel so that it is less prone to defects and more stable. This can also prevent or eliminate any wobbling of the wheel.

Re-tighten spokes – center the wheel:
 Re-tightening the individual spokes on a wheel so that it is less prone to defects and more stable. This can also prevent or eliminate any wobbling of the wheel.

Relevant articles:
Spring check bicycle , repair bicycle tube