Self-plagiarism is often described as the reuse of one’s own previous work without acknowledging that you are doing so or by not citing the original work. It is “recycling” one’s own work without referencing the earlier work.
Copying and reusing sections of your own writing would be self-plagiarism if you don’t cite the earlier work.
If you are reusing short passages verbatim, they should be enclosed in quotation marks and clearly cite the previous publication. Long passages should not be reused verbatim.
Text Recycling Research Project.
This NSF-funded initiative is a great place to learn more about what is and is not appropriate practice regarding re-using your own material.