Stability is thermodynamic; folding is kinetic
The native state is the conformation with the lowest free energy under cellular conditions, so the folded protein is favored at equilibrium. That thermodynamic fact does not tell you whether the protein will actually get there. A polypeptide must traverse a folding funnel in which partially folded intermediates expose hydrophobic side chains that are normally buried in the native structure. Those exposed surfaces can associate with each other, and because the resulting aggregate is often kinetically trapped, the cell can end up with a stable but non-functional deposit even though the native state is thermodynamically preferred.
What chaperones actually contribute
Chaperones bind exposed hydrophobic segments and shield them from inappropriate contacts. They do not push the polypeptide toward the native state by adding energy; they reduce the competing aggregation reaction and give the client repeated opportunities to fold. Because binding is stoichiometric and transient, the number of available chaperone molecules sets a ceiling on how much misfolded client can be managed at once. When that ceiling is reached, the excess client is left exposed, and aggregation becomes the dominant outcome.
The capacity limit is the central variable
Every later section of this chapter is a variation on one question: what happens when the load of misfolded protein exceeds the capacity of the machinery available to handle it? Chaperone-assisted folding is the first line of defense, but it is saturable. The next two pages describe the two degradation routes the cell uses when refolding fails.