I do not intend to define this word exactly, nor am I certain of the exact definition. I understand, and believe others do also, that it is derived from carrying and leaf. Linguistic scholars are welcome to comment. My sense is based on ~ porters (who carry baggage) ~ portages (when canoeists carry their boats) and foliage (on trees) ~ folios (of folded paper). I write notes here on storage or carrying cases for loose papers - a form of filing system.
Making portfolios is an excellent part of any bookbinding course. Since the binding part of that sentence is rather debateable, it might be better to say it is a useful bookcraft skill.