It means that there has been a slight genetic change so that the plant produces something slightly different from the original. In this case the change is the colour. Treatment is exactly the same, as to all intents and purposes, the plants are the same thing.

A sport is a variation that has happened during vegetative growth rather than from a reproductive seed. Although these can be a slight genetic (DNA sequence) change they are also likely to arise from epigenetic changes, that is a change that does not need a difference in DNA sequence. Epigenetic changes are more often reversible whch is why you so often see reversions in sports. Examples of this include DNA-methylation patterns and prion proteins.
Viruses, fungi and transposable DNA elements also give rise to apparent sports, such as Tulip Break that is caused by a virus.
Sports can be weakened by the changes but not always and usually are treated the same as the originating plant.