Self-plagiarism occurs when you reuse your own work. Below are common examples:
1) A student submits their own paper in more than one course without permission of the instructors.
How is this plagiarism? An important part of academic honesty is that your writing should reflect what you learned in a specific class. The point of college is to acquire knowledge, and your education is an investment in you. Don't cheat yourself.
2) Someone uses their own work (either a section or the whole thing) that was previously published in a different publication without properly citing it.
How is this plagiarism? If you don't properly cite your own previous work, you will give off the misleading impression that your recycled work is actually new.