What is scaffolding? Is it a term for a particular platform?

Scaffolding generally refers to a quickly set up skeleton for an app. It's not rails-only since other platforms have it as well. It's also not generally meant to be a "final" system; merely the first, smallest way to do it.

From Wikipedia:

Scaffolding is a meta-programming method of building database-backed software applications. It is a technique supported by some model-view-controller frameworks, in which the programmer may write a specification that describes how the application database may be used. The compiler uses this specification to generate code that the application can use to create, read, update and delete database entries, effectively treating the template as a "scaffold" on which to build a more powerful application.

Just like a real scaffolding in a building construction site, scaffolding gives you some kind of a (fast, simplified, temporary) structure for your project, on which you can rely to build the real project.

It can be (and is today) used to describe many things - from abstracting DB layers, to web apps folder structures, and to generating and managing project dependencies .

It is not something that is specific to any language / technology, just like the term skeleton or boilerplate is platform agnostic.

It is just a term borrowed from real scaffolding (like mentioned above). You build some fast, simplified, (sometimes external, sometimes temporary) structure that will help you to build the real, more complex, finalized structure under, above, inside or outside of that temporary structure .

.. And just like the real scaffolding, the scaffolding structure is meant to support the building process of the project, rather than the project itself (with some exceptions).

Scafolding is usually some type of code generation where you point it at a database, and the technology creates basic CRUD (create, read, update, delete) screens.