Like its physical namesake, scaffolding in a software system is a functional but cookie-cutter user-interface that is less attractive and robust than actual construction. Just as in construction, scaffolding is often used as a temporary solution while the actual construction is going on. Unlike the physical world, it is possible in some cases to modify the scaffolding to eventually become the "actual" structure.
In practice, scaffolding is a system that generates code based on an object model or data source. It was made popular in web development after Ruby on Rails was released with an easy scaffolding system. Modeled on Rails, Grails also includes scaffolding, while Microsoft created Dynamic Data, a framework independent scaffolding system.