[COGlory]

To expand on that (great explanation btw) and to get at something the original question was asking:

A protein can have many different, stable conformations. Those conformations depend on the chemical environment, and any interactions the protein is making. Basically they alter the lowest energy to be a different arrangement.

However, basic elements of the fold, with a few exceptions, will never change. We call these secondary structure elements, and they are limited by phi and psi angles on the dihedral C-N peptide bond. These secondary structure elements are thought to form before the protein is even fully synthesized, and are extremely difficult to undo. However, the spatial relationship between these elements is much more dynamic depending on what the protein is doing.

The ELI5 version is basically that proteins will have a basic shape, and they can wiggle around that shape, but can't really radically change because it would take too much energy