the framing

This to the panels that they made by joining together smaller pieces of wood.

crossbars

These are support beams that give stability to the structure.

A post is a strong piece of wood set upright and used as a support.

The bases held the posts in place.

This means everything used to join the crossbars, posts, and bases together.

Here "their" refers to the "pillars and posts."

These are all items used secure the pillars and posts into place.