Scaffolding is created by three sticks and three planks. It can be reinforced by right clicking (in world) with 2 Sticks on it. which lets it support more scaffolds on its sides.
There is also a stronger variant made of Iron Plates and Iron Fences. This variant can be reinforced further by right clicking with an Iron Fence in order to let it support even more scaffolds on its sides.
Both types of scaffolding can be climbed as if it were a ladder.
Right clicking places one scaffold, left clicking on a scaffold places one scaffold on top of the scaffold you clicked on.
More scaffold can hang off a tower of them, but there are limits to how far they can be supported.
Stick Reinforced Wooden
Fence Reinforced Iron
Wooden scaffold can also be used on with a CF Sprayer to replace them with Construction Foam. Doing this drops the scaffolding back.
To reinforce a scaffold, right click it with an Iron Fence post (for Iron Scaffolds) or 2 sticks (for Wooden Scaffolds). They must have a solid block (such as a dirt block or another scaffold) below them in order to be able to do this.