Self-plagiarism consist in reusing one’s own work, typically one’s own text. Although it is not stealing someone else's work (plagiarism), can still be considered academic dishonesty, for instance, if a student reuses parts of an assignment that they previously got credit for.
Trying to get multiple rewards (academic credits, for example) for an effort that was only done once will be considered dishonest, unless it is done in an honest and transparent way.
Self-plagiarism can be avoided by citing one's own work, even if one only revisits certain ideas.